The volume controls are indeed pickup-based: "treble volume" means "bridge pickup volume" and "bass volume" means "neck pickup volume". This is the same as the traditional Les Paul control setup, except that the knobs are positioned in a different order. RIckenbackers then add the mysterious fifth knob, which is really nothing more than a second volume pot for the neck pickup in series with the main neck volume knob. Even though it doesn't really do anything that you couldn't also accomplish by twiddling the neck volume control, I use it all the time and tend to leave the main volume controls turned up all the way. Plus, it looks cool.Anyway, I really enjoyed playing the 330 -- it felt good and the sound was really nice -- but I struggled a bit because I didn't really understand the controls. They're labelled (Treble, Bass) + (Volume, Tone), but the volume controls seemed not to be freq. band based but rather pickup-based, which seems odd.
That's the easy answer. The tougher problem is your dilemma about which guitar to buy. It sounds like the 12-string has really grabbed you, which is completely understandable. Nothing else sounds quite like it. Most of us have had similar experiences and, not coincidentally, many of us have wound up owning Rickenbacker 12-strings. They are more versatile than is often thought -- a lot of stuff that you wouldn't ordinarily associate with a 12-string can be played on a 12 and sound very good. However, an electric six-string (Rickenbacker or other) covers a different range of sonic territory, and there are there are some things that can really only be done on a six. For instance, you really can't do string bends very successfully on a 12.
From one perspective, the only satisfactory solution would be to get both, but if you're on a budget this may not be feasible for you. Much depends on your priorities. If your main goal in getting an electric is to learn how to play lead, you really need a six-string. However, if you have really fallen for the sound of the 340-12 and aren't concerned about trying to replicate the six-string sound, then the 340-12 might be right for you. You can play blues on a 340-12 and sound good -- different, but still good.
Assuming you can't afford to get both the 340-12 and a Rick 6-string, if you've really fallen for the 12 and are getting a very good deal on it, one option (if your budget can handle it) would be to get the 340-12 and supplement it with a cheap six-string, like the Squier Classic Vibe Tele someone mentioned downthread. Musicians Friend has new ones advertised for $350, and you might be able to get something comparable used for even less. This would probably be a fine six-string to start out with, and you can save up for something better down the road (either a Rick or something else). Rickenbackers hold their value very well and demand generally outstrips supply, so if if you change your mind later and decide that the 340/12 isn't really what you want, you should have no difficulty reselling it and getting back most or even all of what you paid for it.
I could be wrong, but, again, it's my hypothesis. 
