Tough decision for sure, I balanced cost vs. speed and went with the 550. My first press beyond the single stage and I liked the control I felt.
I am happy with my decision, but I know many swear by the 650, especially with a higher volume.
they are definitely getting harder to find.
try these guys:
http://www.iidbs.com/hitech.zkb?root&GENMENU6004&object-menu6
I ordered CCI 500's from them yesterday. They were out of 400's.
Check out this post, also do a search on here, there was a discussion in the last few weeks about this.
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