Doug, March/April issue of Rifleshooter magazine has an article about spotting scopes by David Tubb. I see in the picture here he uses Optolyth 100mm w/angled eyepiece for prone shooting. Looks like a good article, check it out. Jay
From a hunting standpoint only, angled can be more pleasent to use while sitting with short light backpacking tripods but for stealth perposes and laying on the ground trying to keep a low profile the straight is better.
Keep in mind this is with hunting in mind not range time.
It all depends on the position or the shooter. A crook in the neck, or moving too far from your shooting position while viewing in most positions is what you want to avoid, how you do most of your veiwing will determine which is best. The straight ones ore nice to get lined up on a target quicker before stuffing your eye into it though. The angled ones don't require as tall a tripod you'll want with using the straight one. I like a tall tripod with any of them though, just to be safe, you never know how tall the brush is going to be unless you hunt the same spot all the time or do it from a bench.