Two equal length without a doubt.

Change of scenery and a break at the half way stage, with food, drink, smoke or whatever.

Can sleep 3 or 4 hours twice if I want.

Depends on the cabin.

Long and a short or vice versa in J particularly if overnight is required.

Equal length if in Y to get out stretch and recharge.

I have flights over water: SIN-SFO, SIN-FRA. Because all of them are in Y, I rather prefer to split them evenly and have 2-3h connection time in between. For this reason I prefer SIN-NRT-SFO or SIN-TPE-SFO to SIN-HKG-SFO.

i used to insist that i break up a 24 hour journey into 2x 10 hour flights: southern africa to east coast us.

then one time i was booked jnbjfk (15.5hrs nonstop) and i was all like angsty before hand.

but i had a lovely flight that i slept on. then i got off the plane in nyc and was happy. one flight and i was there.

so ya. nonstops.

or shortest first then longest last. nothing's more angst-worthy than lots of flying and one final 2hr flight.

I would prefer the latter especially if it means a widebody and/or better meal service (e.g., full service vs snack/refreshment) due to the longer flight time.

Years ago, I had the option of SJC-ORD-BOS or SJC-LAX-BOS when the SJC-BOS wasn't feasible. The latter flight offered a better meal service and longer sleep time.

3 and 11. You can get a good sleep on the 11 hour flight. The other is too short to matter.

I'd have to take direction of travel and time of day into consideration

for US west coast to Europe, I'd try to get as long a leg as possible during more-or-less-normal night hours ... this summer we did 9.5 hours SEA-LHR (lv 2115 ar 1500) and SEA-LAX-AMS (lv 1100, layover 1400-1655, ar 1215)

returning in daylight, I'd favor a more even split -- MAD-JFK-SEA (lv 1015, layover 1230-1600, ar 1900; 7.5 hrs and 5.5 hrs) was rather easier to take than LHR-SLC-SEA (lv 1245, layover 1630-1930, ar 2100; 10 hrs and 2 hrs) -- however, JFK is far from my first choice of connecting airports under any circumstances, and in terms of ease of CBP entry I'd look for nonstops to SEA regardless of flight duration

I like the longer flight to be as long as possible, but I also hate to arrive so early at my destination that I cannot get into my hotel room for many hours.

Add me to the one long, one short camp.

IMO, back to back 6-8 hour flights is the absolute worst flight combination that exists out there.

Thankfully it's not a common option to anywhere from the US East Coast (closest I come to it is - TCON and then TPAC vs. flight to East Coast/Midwest Hub and then TPAC where the latter is always preferred) but I know it's a particularly relevant question for those transiting between Europe and Asia (and v.v.) and choosing between ME3 and legacy Europe/Asia carriers.

I hate this big time too. There is nothing worse than hanging around in a hotel lobby at 8am after a red-eye.

It's easy enough to avoid. Book on-line for the night before. Either you will get a room immediately or they will refund the first night because you arrived before the check-out time. Yes, it does cost more for the extra night but after an overnight long haul it's worth every penny.

I tend to plan my travel these days to avoid those awful early morning arrivals. The times I do get stuck with them, I have generally found most IHG hotels to be quite accommodating with either checking me in early, or shutting me up with a seat at the breakfast buffet.

It would be a fight for me to get reimbursement for the extra hotel night.

at some point it would seem to make sense to pay it anyway and deduct it as an unreimbursed business expense

I do a lot of that anyway, but I prefer to avoid subsidizing my employer more than necessary.