Why am I drawn to the aluminum??

It comes down to personal experience in terms of the respondants here. Here's my thoughts... Iron is cheap. That's why it's used. It's heavier, harder to repair, and harder to port/modify. that's not math but simple fact. Look at any production car or a large percentage of racing cars - in any class, any type of racing. If it's not mandated it's either a labor of love or a cost issue to run an iron head. It's not that iron makes any special power. It's cheap.
The iron/vs aluminum in terms of compression is a load of bullshit. The builder knows (or should) what heads he's using when they start the parts chosing. So it's a moot point.
In terms of initial costs RPMs are sold complete for $1550/pr. They will need a good valve job and possibly springs. I have what I'm told is the most expensive shop in the world doing my work - it costs me another $450 for that work. So I'm at $2K plus shipping. IMM Indy/RHS are sold for $1469. In my opinion - having a local shop try to do them up will cost more. I know it would for me. Indy/RHS are sold bare - so you have to pay for the parts and labor and that will include the valve job. BTW - It's the valve job that gives the flow results. Both in terms of IMM's work and where the factory claims for RPMs' flow. You won't find an "out of the box" flow for the Indy/RHS - because they have no valves "in the box". In tems of flow figures - again - as a builder I know what I need to make "x" horsepower with "y" amount of flow. In terms of "non ported heads" the RHS and RPM are close to the same ready to run. Certainly close enough that a camshaft choice can even up the outputs of an two engines built similarly. So much of individual results are based on the flowbench setup that to me again - the differences are not that big of a deal.
So - it comes down to weight, bling, and cost. 50lbs afects ET more in a slower car but we're really only talking about .05 and 1mph. the horsepower should be the same regardless of the head choice unless the builder is restricted to using identical camshafts. But ask any road racer, or for that matter anyone that drives "spiritedly" daily what they can feel. I can feel 50lbs in my car. Be it the 5gal bucket of sand, a replacement bettery for my truck, bags of salt or feed in the trunk, or someone on the passenger's seat. It also affects fuel mileage and braking to a limited extent. I can feel it, and comparing regular driving to track time - I'm on the track .002% of the time.
Bling - iron's got no bling. Period...lol.
So totally broken down - in my experience - The difference comes down to $500 and road feel. If I have a customer who's going to spend $5500 on an engine then $500 is less than 5% of the budget and isn't a big stretch.