The UO Outlands staff have done an incredible job of balancing end game templates here. Both of those templates can easily haul in the gold. Dexer, Bard, Tamer and Mage are all pretty evenly balanced and which one happens to be slightly on top (regarding GPH) really depends on what was patched most recently.
Try to look into other differences each template has to offer and determine which is more important to you. GPH is not a good indicator of which template to play unless you want to chase FOTM templates ever patch. In which case, no one knows how this taming/summoner overhaul is going to land, so to answer the FOTM question we need to wait a little bit longer.
Definitely a good idea. If you want to progress towards end game PvM you will want to stick with that one character and unlock echoes if you ever feel like switching it up in PvM.... so keep that in mind. But you will eventually invest in tomes, codex, aspect suit, chain, and chain links. You will really want to make sure your doing this all on a character you enjoy playing.
I always recommend messing around on test for a couple weeks. Play the different templates, see how they all operate, get a basic understanding. Rather waste a couple weeks on test doing that than months on live.