ok, figuring out which cable is which was easy. the new one is rubberized. old one is slick.
chg'd from about 50% all night and was at 100% by morn using new chgr and new cable (complete bs IMHO).
any usb should work equally as well.
Charging via USB is super handy for cross-compatibility, but the fact is the amperage isn't the same for all chargers. The different results from your two chargers makes absolute perfect sense, because the new charger puts out a max of 1.35a, while the first gen charger puts out 2.0a. The new Nexus 7 is probably only going to charge at a max of 1.35a (device regulates amperage draw, not the charger), so it will charge at the same rate on either charger. However, the first gen N7 will try to draw 2.0a, which it can get from one charger, but not the other. The result is equal charging speeds for the new N7, but slower charging for the old N7 on the new charger.
If you need multiple chargers, and want to be sure you're getting full charging speed no matter what you plug in, get a higher amperage charger, simple as that. I have yet to use the charger that came with my new N7, because I already had some 2.0 & 2.1a chargers around. I've also been burned before buying a portable battery USB charger, only to find out it has a max output of less than one amp. You can still charge from something like that, but it's sloooooow - so slow you essentially have to turn off the device while charging, or the usage of it being on will exceed the charge it's getting from the battery charger.
Also, *most* of the time the cable you use won't matter, but there are exceptions.