I can't speak directly to your two choices, but I moved from Infinity to Martin Logan.
I had Infinity 4.5 RS and the external crossover using two hafler 500's to power them. I had that setup for years and years. I was in Seattle with time to kill and went into a store that had the Aerius setup. I had a fond recollection from the 70's of an Acoustat 2.2 demonstration playing Vivaldi so asked to hear them. I was immediately struck by the transparency and soundstage coming from the small size of the speaker. I was not thinking of replacing the 4.5's but that clean demo and Acoustat recollection pushed me over the edge and I bought a pair of ReQuests to replace the 4.5's.
The ReQuests were cleaner, better at micro-dynamics, and more involving at low power and "normal" volume. The Infinitys were better at high volume and bass impact, power, and control. Both compressed at higher volumes and had bass discontinity but the Infinitys were better at hiding this trait. End result, I kept the ML's and sent my brother the 4.5's for his birthday.
I now have Summits next the ReQuests in the music room and am hoping for more and better integrated bass impact and some more volume before compression. (Just bought the Summits and haven't hooked them up yet). I use tubes now for both pre and power and Logans are more "alive" in my opinion using tubes.
So, you have some excellent Infinity speakers and have your eye on an excellent replacement. It really is a tough call and if you listen to say, primarily Slayer or Zep at high volumes most of the time, I would be inclined to stay with Infinity. As I have aged and started getting into Jazz, acoustic, alt-country, punk folk, etc at lower volumes than say "Live at Leeds" tripping 10 amp fuses in Haflers I find the Logans exceptional at lower volumes with less complex music.
One factor also is I used to send my EMIM's to Infinity to get re-done and could order replacement EMIT's. Near the end of my ownership Infinity stopped supporting and repairing and replacing drivers. With Logans replacement parts should be available for quire some time.
You have a nice quandry. I really enjoyed my Infinity's but I still feel that the Logans are a definite step-up.
Good Luck and Have Fun.
