I don't have any direct hands on experience with the planar 25 or the older rp3 but I have owned a new model planar 2 (with both the rb220 and rb330 tonearms) and I now own a current model planar 8. My Dad has a current model planar 6.
The current model planar 3 can be found open-box without a cart for around $799 . I have seen a few of the older rp3's for $699. Between the two I would absolutley go for the current planar 3. The rb330 tonearm on the newer planar 3 is a fantastic arm and made a very large, positive change to my planar 2 when I upgraded from the factory installed rb220. That same rb330 arm is used on my Dad's planar 6. I also like that the main bearing housing on the newer models is actually threaded into the plinth as opposed to being secured with a nut on the underside. And it uses the newer 24 volt motor that can be paired with the external neo PSU speed control box if one ever wanted to add that down the road.
I feel like a lot of people could live with the newer planar 3 without making any changes or upgrades and be just fine long term. That being said, of all the changes I made to the planar 2 when I owned it, 2nd to the rb330 tonearm, it was the groovetracer reference subplatter and zirconia bearing that made the biggest difference. The noise floor dropped and the bearing rumble from the planar 2 disappeared.
Good luck on your turntable hunt.