757 is as you say, only LBA or MAN based. It does have the advantage of being over 100T so if you have aspirations of a direct entry command in the sandpit then a command on the 757 will give you relevant experience.
If you don't care about the gulf or want to be based somewhere other than MAN/LBA then take the 737.
By default, due to the numbers, the 737 has a greater range of destinations.