To my knowledge, the only aftermarket aluminum block is a Dart block. Anyway, I think what you are asking is if an aluminum block would be worth it over an iron block. If so....
My opinion is "no," especially if it's a street car. Aluminum expands and contracts much faster than iron (obviously) which means that quite a bit of extra work needs to go into prepping/machining an aluminum block for street duty. The block needs to be sleeved, which I think costs quite a bit.
You will get the people who tell you, "well Mazda, Chevy, Toyota, etc. all put aluminum blocks and heads into factory production cars." And that arguement seems great. My question to them is this: "if there are no differences in iron and aluminum (which the above arguement is basically saying), then why don't more people use aluminum blocks in serious, high performance street cars?" It's not as simple as people make it out to be.
I'd say use the iron block ONLY because the aluminum one is crazy priced. That to me is the reason you don't see guys use them for build up's. If I remember, its $4000 for the block alone. I may be wrong, but its somewhere up there.
Plus, how much weight would you actually save? The extra $$$ you spend for the alluminum block, could easily be used for a new k member, fiberglass parts, etc etc and make the car even lighter then an alluminum block. Or just swap a MAch 4.6 engine in, 4 valve performance and its alluminum.