I need to take a Shuttle LGA-BOS-LGA next Sunday (8/25)AM and return late that PM. The fare for a seven day advance purchase seems to be the same $175 on both airlines.
A - Which one will give me the most miles?
B - Is there another fare that I have not found?
Other factors which may or may not be important:
1 - I am Gold Elite on both of these (from Amex) although I have not used either for a couple of years and have not been at BOS for about 20 years
2 - At BOS, does each of these companies have equal access timewise to the rental car pickup areas, or is there one rental company that is considered to be more strategically located than the others?
3 - All things being even, a small factor would be the proximity of a President's Club or WorldClub to the chosen company's gate area
4 - ATBE, an even smaller factor would be the (barely) cheaper cab fare to the Marine Air Terminal at LGA
5 - I am a member of AAA
6 - I have cross-posted this on both boards 7 - It really goes without saying, but I will make every effort to help with a similar question about CO should one arise

I'll take a crack at some of your questions:
1. At BOS and LGA shuttle facilities for both airlines, you will not be able to access a PC or WC without changing terminals which would take you a while;
2. Fares: There is a promotion with Delta Shuttle and Duncan Doughnuts, not sure when it runs out, but basically you buy coffee beans, and you get a cert to buy a weekend RT for like $87. I believe you must buy the tkt at least one day in advance, etc. If you do a search on the DL board for shuttle or Duncan Doughnuts, you should be able to track down some information on it. The last flight you can return via this promotion may be like the 330PM or so, but if you think you can talk yourself on a later flight standby try it if your in to that. Your luck will vary, but it could be done for you, or maybe not.
3. Miles: Both airlines are offering tripple miles, and I know US just extended this to 12/31/02. DL will liekly follow suit, although both are definetely going on until 8/31. What airline will you fly for other trips in Aug and Sep? US on balance has more bonuses right now, see the Bonus Code thread, but if your more a DL person you can sign up for either 750 miles per segment or double miles;
4. Food and beverage: During non lunch and dinner flights, Delta has hands down a better snack service. They offer a snack basket with cheese and crackers, vegtables and dip, a chocolate, etc. DL also still has bottled Sam Adams beer, while US just has cans. If you fly US on the weekend you will just get pretzels or peanuts or not the greatest one's at that. US during the week, during lunch and dinner flights, does offer a nice delli sandiwch snack that you pick up from the fa when you board. But again Delta wins hands down for off peak food and beverages;
5. Entertainment: US during the business week offers Fox News Now, a twice daily updated news service on your headset which you pick up free at the boarding door podium out of a basket. On the weekend, US will also offer some audio, but not sure they keep the headsets out, probably? I am not sure that DL offers any kind of entertainment system, though the jacks are present. Both shuttle have Empower laptop ports, but you will need a special adapter to use these which if you have one you know it, and if you don't you don't.
6. Boston Terminal Locations: Both the DL and US Shuttle are now operating out of the terminal B side of US....AA and one or two others are in a different part of term B. US has newer and more spacious facilities, and just opened a shuttle concourse last summer. As you enter the upper level of the terminal, with the doors to the street at your back behind you, the US shuttle is all the way to your left, and the DL shuttle is all the way to your right. With in the US security area, you can access the US club and a legal seafood and a food court, part of the mainline concourse. Go through the shuttle security line (faster) and then turn right and go through a hallway which will lead you to the food court, and gates 4-13. The Legal Seafood is down the gate 4-13 concourse on your right. You might think you won't have time, but maybe you'll get stuck in BOS due to ATC and then you'll perhaps want to avail yourself of drinks and food at Legal's. Shuttle concourse is gates 15-21, and all gates are on the right hand side except 20 and 21 are kind of at the end of the concourse.
I have no idea of which LGA terminal, Marine Air or US, is closer for rental cars or taxis.
Hope this helps.

A few more things...
DL operates Boeing 737-800, while US operates A319 and A320 aircraft. The Airbuses have cloth covered seats, while the Delta Boeings have leather. On DL it is open seating, while on US you have assigned seats. Also US has a 1,000 first eticket check in kiosk bonus, and 50 miles per subsiquent check in so if you go with US use the kiosk. I don't think with DL that you can use VCI (virtual check in) either phone/web/pda for shuttle operated flights.

They are both pretty similar, and the flight is short enough so that a marginal difference in quality isn't going to mean that much. The A319/320's are supposedly a few inches wider.

Seats, seats, seats!

This is the ONLY reason I choose US over DL (until the UA code share kicks in, then I'll have 2 reasons). US gives you an assigned seat, for DL you fight for it. I HATE not having an assigned seat. DL loses my business on this. The cattle crush on DL at the gate of New Yorkers is pretty scary if it's a full flight http://www.flyertalk.com/forum/biggrin.gif I do the BOS-LGA shuttle twice a month, every month. Role on UA code share!

However, the DL shuttle at LGA (Marine Air Terminal) is much closer to Avis and Hertz than the US terminal. You can (and I have) actually walk from the DL shuttle terminal to the Avis lot.

Thanks to all for the very useful info. I have decided on US, taking the last airplane back on Sunday. As a note, the Dunkin Donuts promo is only for New England residents originating in BOS.
It seems that with online booking, AAA bonus, triple miles and e-machine checkin, I can pick up in the neighborhood of 5000 miles for this quickie roundtrip. Not at all bad, and it even rounds my long unused US miles account up over 50000, which has got to be some sort of award plateau.
Also, I am assuming that if I get back to BOS
that evening in time for an earlier flight, I can standby and go if there is an opening.