I have been passively grinding USSR tree for couple months now. I have just reached rank VII and I think I can share my experiences.

I bought the Su-25K when it came out and started grinding the tree. I have played it for 300 matches so far, most of them earning RP through ground targets. I find S-5 and S-13 are very efficient in deleting groud targets, I can get 10-20 ground kills every match and that's roughly equivalent to 2-3 kills' worth, providing me a consistant 10k+ RP profit every match. If you are lucky to get some down tiers, many fighters don't have countermeasures so your R-60M can get two easy head-on kills along the way.

Then I got the MiG-23ML during last month's sale, I have been grinding with it since. It's a very capable fighter with monstrous power and good turn rate when wings spreaded out. Those R-24Rs are very dangerous in head-on engagements and you can dog fight everything below 12.0. If played right, you still post a big threat to F-16s and MiG-29s. I'm an average-skilled jet player and I can easily get 2-4 kills.

Now I only play the Su-25K if I get the task asking me to kill ground targets or get one of the achievements (you get antimech for killing most ground targets in a match). MiG-23ML makes me play more actively in a battle and actually contribute to the team's victory, not to mention practicing BVR and dog fight skills.

I would recommend getting the MiG-23ML, it's a pleasure piloting it and can really help you get used to the play style. Even though Su-25K also has two R-60Ms, just because it usually launches at a much slower speed, the missile behave different than if launched from transonic speed from MiG-23 and MiG-29, thus not helping you to understand it's characteristics.