I can't decide which flight I should take on a trip from Boston to Las Vegas. Basically I want the flight to go by as quickly and smoothly as possible.

I'm picking between 4 choices:

Delta - 3 of 4 legs are on 764's, where I can get a projector aisle (33) with unlimited legroom.

Delta - same as before except 2 legs are on 738's. Are these more comfortable and worth not having an open aisle?

American - MRTC on 3 of 4 legs on MD-80's. Sixty bucks more. The flight time is 40 minutes less.

United - 3 of 4 legs are on A320's. 18" width seems great, but Seatguru warns against the coach seats. Not sure why. This one is the shortest flight time at 70 minutes less than the Delta and 2 hours less total trip time. 50 bucks more than the Delta flight.

As a bonus question, for a trip to Vancouver, is it better to take Air Canada with a 1 hour flight each way on a CRJ and A320's the rest, or United with A320's the entire trip?

Thanks a bunch everyone

First, welcome to Flyertalk!
IMO, and seeing that you stated that you "want the flight to go by as quickly and smoothly as possible", I would go with United. IMO, UA's A320's are a good, comfortable aircraft. I am unsure why seatguru states that "There are complaints about the coach seats on this plane being more uncomfortable than most others". They are relatively new, clean, and I have never personally had a problem with the seats.(Although I have never been in E- on this aircraft.)

For your bonus question I would go with UA again. I personally try to avoid CRJ's if at all possible, (can't carry on anything larger than a Readers Digest); and regardless of what anyone says about the A320's seats, I'm sure no one would dare say that they're worse than a CRJ!
As it may also depend on time, price, etc. for you- and if you do choose the CRJ, an hour is fine- probably just about enough IMHO.
Hope this helps, and again, welcome to FT.