I bought both the 10" & the 12" buckets.
I can't say anything about the 10" bucket because I haven't used it yet, but the 12" bucket is really nice: it removes a good amount of dirt and has a lot of digging power.
Also, the trench it digs is baaaaarely big enough for me to squeeze into sideways (Got Diet?). But my wife can stand facing in-line with the trench, which is handy when you need to clean it up. My kids run around in the thing, chimneying up the sides and so on.
I doubt I could fit at all in the trench dug by the 10" bucket and I doubt my wife would find it at all comfortable.
I got the 10" bucket because (and I may be wrong here) my understanding is that because it is 20% wider, the 12" bucket will have 20% less digging force. Around here the dirt, mostly decomposed granite, gets unbelievably hard when it dries out, so I'm anticipating using the 10" bucket in the summertime and fall when we haven't had any rain for many months.
If I'm wrong then I will soon have a very expensive planter for flowers, slowly getting buried in the dirt...
Hog