Is it better to use the unfilled, empty cartridges that they sell on the V2cig website? Or, is using the old prefilled cartridges okay?
It is always better to use new empty carts when compared to refilling the prefilled ones. Refilling the prefilleds WILL work (I got thru my first month that way), but other than the menthol cartos could never find a juice from another vendor that matched as well as the original flavor.
When I first started, I had little money (blew over $100 on the V2 and accessories). So I started small, getting a few cheap juices here and there (watch the "Deals and Steals" thread in the PIF section, good coupon codes there daily).
Used those juices to keep my V2 cartos going until they were all hozed (happened sooner than I would have liked, because in my quest to "use" all of the stock juice in the cartos, most of them singed).
In the meantime I was saving my can money (5 cent deposit in Iowa still) for some empty cartos and the Riva SE kit I needed for the battery life (was quite happy with this purchase). Got 10 Boge cartos at the same time, all empty, and started filling those with my first bit of liquids.
To your specific question about the empty V2 cartos, they should be fairly similar to other KR808 cartos (more or less). Many swear by the 808 cartos at V4L, some really love the Bloog. I personally like Boge cartos, since you can get a range of resistance (2.0, 2.5, 2.8, and 3.0 for certain). The Riva I picked up has a 510 connection, rather than the 808 threads on the V2, that's why I went with 510 cartos.
Boge does make 808 cartos, but usually only the 2.8/3.0 models can be used on the 280mAh batteries that come with the V2 (current draw too strong on the lower rez cartos, another reason a fat batt like a Riva/Ego is handy).
The $8.95 for empty V2 cartos isn't really a bad price (shipping is a bit high though), so sticking with these should be fine, plus it will keep your battery warranty current (if that is important to you).
Last time I picked up some juice on a super sale, had 5mL bottle samplers for $0.98 each in the Deals thread. A few vendors will give you free shipping after just $15-$20. You can do all of this and still periodically buy some cartos from V2, ah the joys of flexibility!