I made this switch as you can see from my posts in this thread. If you follow suit, you'll pay the same $149 as I did being an existing Metro customer.
I'm very happy with the F3, and for me at least the smaller memory hasn't been a huge problem. Even with the 5GB+ on the Motion I hadn't used much more than what the F3 can hold, and with Link2SD I'm able to stretch it out some more. Other than the memory, everything else about this phone is equal to or superior to the Motion.
Of course your requirements may differ, so if you require a lot of internal storage and you don't think Link2SD is the solution for you (maybe you don't want to root) then this may not be the phone for you. But that doesn't give you a lot of options in the same price range, especially if you want to go GSM. You're stuck with the S3 or S4 at a much higher cost.
You should also find out how good the T-Mobile network is in your area and anywhere else you may be using the phone before making the plunge. I knew going in that T-Mo has been building their LTE up in my general area.