Depends on what you want to do (and what your PSP3000 is capable of doing.)
1) If your PSP isn't a 7/9g you can downgrade to 6.20 for 6.20 PRO, a "permanent" cfw.
2) Since 6.35/6.39/6.60 PRO are all "temporary" cfws, Rydian's guide skipped the 6.35/6.39 versions and suggested going up to 6.60 anyway. All of them probably work just as good, so why not get the highest ofw version. You could always opt to stay at 6.39 and use PRO for that.