I doubt that AA will honor lifetime top-tier status for TWA converts. Most likely they'll get AA Platinum for life. But then again, you never know...

In terms of using-power of 100,000 miles on TWA versus AA, there isn't a clear cut answer. AA has a MUCH better network of award destinations, award partners, and frequency of flights. But awards on TWA can be a *LOT* cheaper. The question really comes down to what do you want to get out of the specific program.

Personally, I'd be leery of moving my AmEx miles into TWA hoping for the AA buyout. It's always possible that the buy-out won't happen.