If you are in the states I would recomend Hammerhead evolution. If the vis is 15-30 feet I think a 90 or a 100 would be very suitable. Probably more like the 90. like I said before these guns are affordable and very well made.
As far as the rear handle vs. mid its all personal preference really. With a shorter gun like a 90 or 100 I dont think its really that critical weather its a rear handle or a forward handle. The low profile of euro guns makes them very easy to maneuver underwater. Plus with the rear handle you get that little bit of extra reach. And lastly I think that if you are going with the more affordable euro style guns the rear handle is the standard. Mid handles or forward handles are more predominant on wood guns. Wood guns add a bit more to the price tag.
Rob Allen and Omer make a few differnt guns. I believe the Omer Cayman (aluminum) is pretty affordable. once you start getting a little technicle like the cayman CF (carbon Fiber) or the Cayman HF ( Elipticle shaped barrel), thats when the prices go up quite a bit.
What every you do... dont go w/ JBL. I know the prices are appealing but the quality of the guns are... OK...im going to stop before I get myself in trouble.
