Level 65 > Level 60.

The differences in the resist ratings are relatively tiny.

The big difference is that you can get a vastly improved Tactical Healing Rating by using a 65, both because of the inherent bonus, and because you can slot the +Tactical Healing crafted relic.

All considered, this adds up to somewhere between 100 and 200 more power transferred per Share the Power. This is significant, and it's why I use a 65 book, and a 65 staff.