Dissenting opinion here. Aluminum heads will allow 10.5:1 compression on pump gas, and to me, that alone justifies spending $200 to $300 more than an equivalent set of iron heads. You also loose 60 to 70 lbs of weight. As for the Performer vs Performer RPM's, I have been running the latter on a sbc in my flatty for almost 10 years and can say there is no downside to not having that heat riser crossover. I suspect the emissions are more when cold but that's about it. I have experienced nary one problem with cold weather operation and it does get cold in Montana.
You will obviously need to nail down what compression ratio you want before ordering. At least for Chevys, there are several cc chamber options along with the straight or angled plug option. For the cost, the RPM's are a really decent buy. I don't know what the budget for this build is, but those heads work really well with a roller cam with steep ramps. You can get a lot of lift with a short or moderately short duration cam. The 381 sbc in my 3B has had a Comp roller since 2008 and IMO, cat's meow.
1955 CJ 3B, 381cid sbc (441cid all aluminum sbc coming soon), AGE M22W, Teralows, Warn OD, OBA, Premier Power welder, Warn 8274
1968 CJ 5, factory 225" w/ headers/450 cfm Holley, T86AA, Belleview winch, Warn OD
2012 Wrangler Rubicon Unlimited, Warn 9.5 xp-s winch