Default New to a One

Im thinking about dropping Sprint and going with Tmobile, and looking to get a HTC One. Only have a few questions about the phone on Tmobile. Hows the speed and Rf performance? Also, i know the phone is HSPA+ and LTE but how dose that work(?) meaning lets say ive got good LTE signal and i go into my work(metal building) will it try and hold the LTE or drop down to HSPA+? I know the new HTC OnePlus per say is coming soon, i may upgrade to that eventually. Currently have a Lg G2 on Sprint and love the phone but not the network. Thanks for the help, im going to think about ti for a week or so.
Samsung Galaxy S5
-Stock no root
Case: Trident Kraken

On the Now "but slow" Network