BGTutu -
Total Conversion mad which makes BG2 into BG1. That means you get the BG1 storyline, NPCs, etc.
AND the BG2 graphics, ruleset, kits, engine,...
BGT -
Mod that adds BG1+TotSC to (before) BG2. That means you get BG1+TotSC+BG2+ToB storyline (connected by a transition cutscene from BG1 to BG2) all in the BG2 graphics, ruleset, kits, engine,...
So basically BGT would allow you to play the whole trilogy as a Kensai for example without saving once (heed the bugs
).
With BGTutu you could play BG1 as a Kensai, save you char, uninstall the mod, load your char and continue through standard BG2+ToB.
However BGTutu is a much smaller download and a lot easier to install.
Both mods require you to have all of the trilogies CDs (BG1+TotSC+BG2+ToB).