I've got a Dirtworker & a mate has got the Mobi one. They seem quite similar in terms of performace, although I think the Mobi stores more water.
His power cord has gone faulty in the cigarette lighter connector and he's only used it a handful of times, but that's probably just an unfortunate occurrence & not something to base your purchase on.
You can get the Mobi (I think) with a rechargable pack, but I don't know if that's loads more cash – I assume it is.
I think the Mobi has a valve on the cap, rather than a load of holes in (like the Dirtworker) to let air in as you use the water. With my Dirtworker you can get a very small quantity of water sloshing out if it's brimmed full, but I seem to remember reading that dirtworker had swapped the holes for a similar valve. My dirtworker is about 2 yrs old now, so it's probably had a few updates since then.