I've always been a buy once person...

What you'll get is a lifetime warranty (Arkel for sure, and Ortlieb too maybe?), much better attention to detail (no seams unraveling, etc), better materials, strength (for heavy loads), customer service (can only speak for Arkel), and better attachment mechanisms.

The same is true of racks. Aluminum might be great, but having the peace of mind of knowing that when I really really put a load on the rack it will be fine was worth the extra money. After all, if every time you hit a bump the thought to check the rack comes up, that detracts from the joy of touring (at least to me).

I knew I wanted a steel rack with a large platform and only two racks fit that...both on the expensive side. I chose the lesser expensive of the two.