Obviously guys are very interested in trying to make the best decision today for what the world will be like 3-4 months after they get hired. If someone wants LAX/LGA/MIA/PHL, and they're not super junior in their class, they can get the base. PHX/DCA/UDC/BOS awards for March went to guys hired in September 2018. Six months, including about three months in training and IOE. So they commuted for three months to reserve before moving to the next best base.
The only options not available to a six month LOS guy right now are ORD/DFW/CLT. The junior award for a March transfer to DFW on the Airbus went to a guy hired 11 months earlier(April 2018). The junior 737 guy took 14 months(Jan 2018 new hire).
Trying to make the 'perfect' decision for a manning decision 11-14 months after you got hired is impossible. And then trying to look 6-12 months later to see what lines you'd be able to hold? We can view what's happening today, and make a judgement assuming the future is the same as it is today, but there's zero confidence in that happening.
A fleet planning chart I just looked at (hopefully still current) shows AG and 737 deliveries in 2019-2021.
If you want to go to a specific base that doesn't have one fleet type I'd try to get the airplane that is based there. For BOS and ORD that's the 737. CLT, PHL and PHX is the 320.
If you think you're going to commute try to commute on line. Next try to commute on AB's(two jump seats). Four AB's flights (8 jump seats) might be better than 6 737 flights (6 jump seats).
And once you're on property you'll have several months to watch how your relative seniority, and the opportunities that develop as retirements accelerate and flying changes slightly in bases, which gives you the chance to switch to the other n/b after 9-15 months (plus up to 3 months lag for the actual award).
I tell guys to expect the first two years to perhaps be a grind if you're trying to get to X base. Right now that's ORD with CLT and DFW as the next toughest at just around a year, or slightly longer. Retirement increases in CLT (86/92 next two years) and DFW (112/156) specifically might generate more opportunities so the lag might shorten.