I've broken 5 Al booms in the last 16 months. Two Loft and 1 NP broke at the head, and 2 Chinook broke in the middle of the arm, just about where the internal plug is.

Other sailors tell me that it's common for Chinook booms to break there, rather than at the head. Anybody know why? Different alloy? Wall thickness?

I've broken about 4 Chinook's, all near the head and all in about 9 or 10 months, I put it down to work hardening (flexing). Having said that, I got the best runs out of the Chinooks, compared to others.
I now have a Chinook carbon, and for me it's proven to be cheaper in the long run, but I only sail ocean with no waves (FNQ).

Just my 2c

having broken multiple aluminium booms i am wondering if anyone has broken carbon booms the same way or are they less prone to breaking. i dont want to spend that much money on carbon if they are still going to break after 3 or 4 sessions like some

Yes last year I bought an Neil Pryde X9 carbon Boom. After breaking a few aluminium booms I decided to give carbon a go. So far it's been great. The biggest problem is once having a carbon you can never go back to a spongy feeling aluminium again. I still have aluminium in the smaller sizes and will soon upgrade. The differnce in my opinion is far more important than masts. It is probably the best single thing to change in your rig to achieve a huge performance increase. Sure they cost more, but it is really worth it.