I was facing the same question. The HK91 that I found was in 98% condition with
no accessories other than one used magazine and was going for $2600.
I found a Stoner SR-25 Match, Springfield Generation 2 optics (4.5 x 14 with sunshade and range finding reticle), 2 twenty round original KMC magazines, Harris bipod and gun case for $2700 (including $100 for shipping and insurance).
When talking to the HK distributor, he asked the same type of questions that Jake98c/11b mentioned in the above posting. What was I going to do with the rifle? Or put another way, what did I want the rifle to do? The HK91 is from a battle rifle background. If it hits an inch or two off, so what, it still got the "enemy" in the center of mass, objective met. If I wanted to shoot competition where 2 inches off is going to cost you big time, I'd go with the Stoner. For three grand, it had better be the Match version and come with a bunch of accessories, preferrable top shelf optics that have the ability to compensate for bullet drop on the .308 168 gn round.
A couple of other things to remember, magazines are a different story for these two rifles. Stoner mags are at least $100 each, when you can find them. HK mags are around $35 (used and beaten up) to $65 (new in plastic wrap).
Ammunition. Both rifles will shoot almost any .308, but if you want bullet holes that touch each other at 200 yards, then your looking at Federal Gold Match ammunition for the Stoner at $18 for 20 rounds, retail pricing.
Personally, I shoot competition and bought the Stoner and have been delighted. Let us know what you decide and how it shoots.
For more information on the Stoner, check out
http://www.biggerhammer.net/stoner/wwwboard/index.html
DblTap
PS: I posted a message in the "Reports from the Range" section of this board regarding my experience with the SR-25. Might be helpful reading.