Is it a bad idea to switch to T-Mobile because it was bought by AT&T?

I'm currently on AT&T but my contract ends next week. I really want the HTC Sensation which is only available on T-Mobile. So my question is, is it a bad idea to go ahead and get that phone with T-Mobile when in probably about a year, T-Mobile will merge with AT&T? Should I just get the HTC Inspire 4g and stick with AT&T? Because I definitely want an HTC phone, no other smart phone. So if I stay with AT&T that is the phone I'll be purchasing, but still..will it be a giant hassle for T-Mobile customers when they do merge? So should I just not bother with it ?