The mAh of those heli batts is a bit low unfortunately

Store them at 60% charge with the rest of your heli gear.

OK, here's a plan...

Go for this radio (it will let you buy more receivers for just 7 bucks each when you need another for another model) and can store settings for up to 10 models, it's a real bargain... http://www.hobbypartz.com/79p-gt3b-carradio-lcd.html

Get a Savage Flux 2350 (no need to spend the extra 150 for the HP, it's more than enough on 4S and you will already have the 2.4Ghz radio above)... http://www3.towerhobbies.com/cgi-bin...=LXABLW**&P=ML (you can get a further $50 off at tower) its the same car as the HP, just with cheaper electronics (4s only, whereas the HP can take 6s, but there will be breakage ) and a cheaper radio (which you will chuck away anyway having ordered the one above), needs NO upgrades out of the box, tires last forever and are good on all surfaces

And some of these from the same site that you got the radio... http://www.hobbypartz.com/98p-30c-5300-2s1p.html They're cheap enough to take away the pain of putting your existing 3s batts away, which sadly, won't do for anything but a 1/16th or smaller.

so for $600 you got one of the best cars in the surface hobby, a 2.4Ghz radio that will see you good for all your surface hobby needs forever, and a set of batts that are ideal for the Savage and a lot of other models down the line too (such as a short course truck if you wanted later). You'll get 90 mins of solid fun from 2 pairs of those batts in the 2350. If your charger can do 10A charging, you'll be able to keep going indefinitely (you'll be able to charge a pair in less time than it takes to run them out).

Honestly, I think I would be remiss to give you any different advice. Having said these items, I'm absolutely sure that I've steered you in the direction of complete satisfaction. Of course I don't know you, and you may be satisfied with something smaller or cheaper, but like I say, with this stuff I can GUARANTEE your happiness.

As for the difference between scales, yeh it's mainly size. Gear pitch usually differs (though less and less lately), but mainly it's the size of the vehicle and the size of the electronics.