I like the 6 banger, but I'm biased since I own one in my 84 CJ7. The V-8 would be nice, but the AMC 8 is spendy to build relative to a chevy.
Building a 6banger up is not a cheap option either, but there's some sick and twisted satifaction wheeling better than someone with a V8, it's more finesse than brute strength. I guess that's why I like the straight 6 over the amc V-8. However, I would rather have a chevy V-8 than my straight six any day. The more GM parts that I put on my straight six, the better it runs.
For example - ignition problems--->GM HEI from their straight six is fully tuneable with a $5-10 spring kit, and $30 for a crane vacuum advance kit....solved all my stumbling idle issues that were present with the FORD Duraspark distributor I had recurved...twice, and could never get the problem solved.
And Howell TBI, which utilizes all GM parts, what a great setup. I flopped my jeep on it's side once, lost all oil pressure for a few seconds, but it was just purring like a kitten, like it did not know which way was up. Of course I shut it down, righted the jeep, checked the oil and so on. I was back on 4 wheels and rolling as soon as I fixed the blown bead that put me there.
And try finding BBD carbs that are worth a poop, good luck, or any of the emissions cluster of junk that came on them in the 80s for that matter. ALL that came off when I put the howell on, now I have a jeep with that and a catalytic converter only, passes smog like a dream, every time. And I can get GM TBI parts at nearly any parts store nationwide. Try getting anything emissions related to the airpump era in a day and your a lucky jeeper. It's never personally happened to me, I always have to wait a day for those parts to be delivered, even in a metropolitan area like Portland, Oregon.
I had 'em both. For off-road slow crawling, I would take the 6. For on the road driving, I would prefer the 8. The 8 got better mileage than the 6, plus sounded a lot better with a set of glasspacks on it.
I've had both in my 5 and I really like the V8. The 258 was a great motor (until it caught fire) but nothing like having the power of the 304. You can never have too much power, you can still finesse a V8, just because the power is there doesn't mean you have to use all of it.