Originally Posted by Larry in TN

If you bid either the 737 or 320 you can bid-up to the 756, or any widebody, at any time. You are only restricted in moving 737 to 320 or 320 to 737 and that lasts for two years.
If you bid 756, you are locked out of both the 320 and 737 for two years.

I'd recommend bidding 737, or 320 if you think you'd like it better, then fly a few months before reevaluating if and when you want to bid the 756 or a widebody. You'll have access to the bid awards for the fleet(s) that you're interested in and will be able to see what your seniority can hold and you'll have a better understanding on what flying for UA is like and what is important to you.

The layovers on the 737 are better than the 320. From EWR, lots of island layovers. Fewer legs per day. More pilots and more trips so more flexibility in bidding. Many in the 320 fleet prefer the 320 because of the airplane and the comfort factor in the cockpit.

Pay is also slightly higher in the 737 fleet. The 319 and 737-700 pay the same lower-rate; the 320 and the rest of the 737s pay the same higher-rate. There's only aroun d 40 737-700 out of over 400 737s and around half (?) of the 320 fleet are 319. That means you'll fly the lower-paying airplane more often and your blended rate (what you're paid for all non-flying activities like guarantee, vacation, and training) is less as it's is a combination of the two rates based on the ratio of high and lower paying airframes in the overall fleet.