It depends really, you can either spend $400 and 12 months learning, or $100 and 3 months learning, and IMHO end up with the same results.
Starting in DBPro, then progressing to GDK is what I suggest - all the tools, object setup stuff, editors, and basic engine could be done in DBPro and it would handle it pretty damn well. It's actually quite well suited to RTS games I think.
Then moving onto GDK as the engine matures would allow things like longer draw distances, more units, but the code format is pretty similar.
So I suggest the big pack the TGC offer, with DBPro and GDK and all the bonus stuff. The difference between learning C++ and DBPro is huge, but going through DBPro to GDK means that you would constantly be able to develop stuff centric to your actual project. If you go straight onto a C++ engine, you have to learn C++ (perhaps after learning C - which is actually not as scary as I assumed), and you'd have to learn the engine as well.
You might have already discounted DBPro as not being powerful enough, but personally I can't think of a better way to develop an RTS quickly and efficiently. You should check out this youtoob of Fallouts RTS project which is dead now, but was a bit like C&C:Generals, and could be the sort of thing your planning to do yourself.
http://www.youtube.com/watch?v=19bzR3cq520&feature=PlayList&p=6EBEED2C67F64799&playnext=1&playnext_from=PL&index=22
