I'd say guys are picking the 190 for two reasons: pay and QOL.

Consider this:
190 reserve is roughly 4-5 months for JFK/BOS at the CURRENT time.
320 guys have been on reserve in BOS for about 1 year and 8 months! JFK has been just over a year.

So...
With same first year pay, you'll make more money on the 190 and second year will probably be similar money (at least in BOS), with better seniority which equates to better QOL. At that point, you can switch to the 320 if you'd like. On the 320, your relative seniority tends to stagnate as people transition from the 190 around their 2 year point.

Disclaimer.
These time frames are based on a quick look at the bid results and are always subject to change.