To use a 3.5 adventure “As Is” I feel it would be useful to have a “3.5 party level” vs “PF party level” equivalent rule-of-thumb.
I get the feeling that one level every 5 would be close. So a 5th level PF party feels like a 6th level 3.5 party. They have a few more feats, HP, skills (from combination of previous skills) and abilities.
Any thoughts?
I'd say the power level is slightly elevated at lower levels, but the same or even lower at higher levels.
Classes and Races are more powerful, but several spells have been reduced in power; attribute boosters are more expensive due to the new slot restriction; and if you use the (optional) limit on buff spells, it'll really hurt the PCs in mid- to high levels.
I suggest 1:1. In lower levels, fights might get a less easier, but not so easy as to be boring. In higher levels, the reduction of magic and magic items really kicks in.