It will all come down to personal preference. I've personally owned 4 shaft driven cars (FT TC3, TB EV0 2, Tenth technology
NTC3) and really liked them, but could not get used to the "snappy" responsiveness of them. I always seem to go back to belt cars (Mission, 414, TA04, MR4, HPI) I like the plush feel of the belt transmission, but again its personal preference.
I'd give the total efficiency nod to shaft though, just because in real world testing a shaft starts out so much more efficient. A belt system starts out a little resistant, but as rpm's build it actually becomes more efficient as the belts tend to lift off the pully due to inertia, minimal contact is made between the belt and pully. Also dont rely on too much of what you read on this shaft/belt debate! I reciently read some rubbish from a R/C mag (nameless) who said exactly the opposite, belts loose efficiency as RPM's build. I have 2 heli's with belt drive tails who's 30,000-40,000 rpm counter the articles report .
As for weight bias, saddle packs will always be the most effective way to balance a car. That doesnt mean you cant balance a car with a battery down the side, you just have to closely weight your electronics, and then the battery, and add or subtract weight from that side that needs attention.