I own both saws. I bought the 500i to run and the 7900 to sit on shelf w/ the other Dolmars. Always been happy with the 500i. I put the WCS fallers suspension kit on it and now I’m REALLY happy with it. After finally running the 7900, I have to say it’s a most impressive saw. The weight and cutting speed of both saws is comparable. The Dolmar required no suspension upgrade, and I kinda like the exhaust note a little better. Well, I pimped out my 7900 (full wrap, HD air filter, US spikes) and ported my 500i.
At this point the 7900 gets the style points but the 500i pulls ahead in performance. If you’re looking to cut bigger wood than you do with your Dolmar, buy the 661. If you think fuel injection is cool enough that you gotta have one (like I did) go for the 500i. You’ll love it for sure, but it won’t out cut my 066. (or the 661).