I haven't used the slightly older models, but it may be worth sticking with them because it's possible to adapt them to use a standard 20v tool battery (I've specifically seen this done for the V8).

Meanwhile on the V10 I have, batteries cost $130, can't be hot-swapped easily, and use a higher voltage that makes the 20v tool battery modification impractical. I planned to replace (and upgrade) the cells on the refurbished model I bought because the battery has been subpar since I received it, but so far I haven't managed to get past the abnormally-tricky plastic clips.