Is there any way that you could try out a couple different impact wrenches to see what you like?
I forgot all about this... but my Snap On dealer gave me an MG 725 to try out for a week before I bought it. I bought it anyway (I was pretty sure that I was going to settle on it), but he made the offer. I suppose I could have returned it for my money back the following week. I was surprised taht he made such an offer, but he was pretty sure I was gonna like the wrench. He was right. If Snap On makes a 3/8 version of the 725, Id consider that in a heartbeat.
Aside from Snap On, the only other name brand that I would really consider is IR.
Now, don't laugh, but for working on my mowers and stuff - and where I don't need a 1/2 impact - I use this little 3/8 butterfly impact that I got from Harbor Freight. It didn't cost much, but it is incredibly effective. In fact, that is the only 3/8 impact I have. Might be worth your time to check out... mine has proved to be a handy little tool:
http://www.harborfreight.com/cpi/ctaf/Displayitem.taf?itemnumber=93100
mine is the 75 ft/lb model, not the 50 ft/lb one.