I recently made the move from 35mm to MF, and looked at Arca, Alpa and Cambo. I selected the Alpa but I was comparing the 2d vs. the STC. I wanted a lightweight solution, which they both are. Like vdault, my reasoning was mostly personal preference and shooting style. I don't think any one item stood out with one or the other that trumped everything else.
- The Arca viewfinder seems really cool, but I don't carry a viewfinder.
- The Arca focus solution is a more robust answer to the shimming problem. You can't shim an Alpa lens. However, I liked the simplicity and efficiency of the Alpa with HPF rings better.
- I liked the grip on the Alpa better. But I must say I don't think the Arca grip was as bad as some make it out to be.
- I'm only planning on carrying two or maybe three lenses: 43xl, 150sk and sometimes the 100hr. If I had a bunch of lenses I might lean more to Arca because of the common helical.
- I plan to backpack and do other significant outdoor experiences with this. The Alpa seemed a bit more robust. Not in how it is built, but in how it is designed.
My guess is not many of the above items are important to very many people. I'm surprised you want to make the switch. You should definitely try it yourself because these are pretty personal issues. Your in Ireland. Go to the PODAS-Ireland and make Kevin bring along an Alpa for you to try. Well worth the experience.