EDIT: Looks like noirelion beat me to the punch
Here's my long winded version anyway
Signal should be the same because as you already know, they use the same towers and i assume the radios in the phones are the same but don't know for certain. If Tmo service is lacking in your area then you'll have bad service with either phone and forget about using Metro towers, the neither model will connect to those.
I cant comment on ota updates as i haven't experienced one on the Metro/Tmo network yet. I remember that Metro was pretty good when it came to updates on their Galaxy S3, releasing them faster than some big carriers. Take from that what you will.
However I've read that while Samsung has released the stock firmware from the Tmo model to the public, the Metro stock firmware has yet to be released for download for some reason. There are users looking and waiting for it as i write this. I understand that the phones are so similar that the Tmo stock firmware works with the Metro model. In fact most roms that run on the Tmo M919 will also run on the Metro M919N. Perhaps that's why they have yet to release the Metro firmware???
Its weird, I dont understand the reason for having two different phones that are essentially the same when you only needed to offer one from the begining. Why cant metro just sell the Tmo version? Is it just about branding and crapware apps?
This is a good situation for crafty consumers though because (i think this is what you want to do) if you can find a lightly used Tmobile M919 S4 for sale, you can buy it for less than the $570 Metro is selling theirs for and go to Metro to connect it, saving you big time in the end.
Either way, they are both great phones.
Hope this helps and good luck!