What about the time to command on the 757?
Depends on how many hours/experience you have when you join. The 757 fleet don't do many hours over the year, the majority of their hours come from June to September, great if you aren't bothered about building hours, not so good if you need to gain hours for command upgrade. The 737 do about double the hours than the 757, so if you join at the same time it will be roughly twice as long to command.