Some things are simply going to come down to yourself as a user. What appeals to me or anyone else for that matter might not appeal to you. Also, only you know just how demanding you'll be of the camera. Just how much do you plan to use it and how hard do you tend to be on your gear?
While I own a Leica camera, I do not consider myself to be a Leicaphile (no one can pick apart a Leica camera like Leica owners, lol). The sorts of things that many Leica owners concern themselves with just don't matter to me. I purchased my M6 TTL new back in 2000 and have been very happy with it. I didn't choose the M6 TTL over any other Leica M models. I bought the M6 TTL because it happened to be the model that Leica was producing at the time I was ready to buy a new Leica camera. Had Leica been making the M5 at the time then that's likely what I would have ended up with. The funny thing is these days I wouldn't swap my M6 TTL for any other Leica M camera even if offered a straight up trade for any other model in new or like-new condition despite the issues that some have with it.
Some of this likely comes down to the fact that I had never owned a Leica M camera before the M6 TTL. For example, I've never found the viewfinder in the camera to be cluttered. The only time I even think about this is when reading the comments of others regarding cluttered viewfinders, lol. Also, the flare in the rangefinder patch is practically a non-issue for me. This isn't a problem all that often and when it does occur I simply shift the positioning of my eye in relationship to the eyepiece (it only takes a minuscule adjustment) to avoid the flare — this occurs at a subconscious level. I've also never found myself questioning whether or not any of my images were degraded by the inaccuracy of the framelines.
Some of it comes down to ergonomics. What I really happen to like about the M6 TTL in particular is the shutter speed dial — both the size and positioning of it, as well as the direction it turns as it relates to the metering system. As this control is involved with every single exposure that I make with the camera it's a big deal to me. I also really like the metering system display found in the M6 TTL. While others might not care for LED displays, I actually really like them. The functionality of the display incorporating the dot combined with the two arrows works perfectly for me. In addition, my needs as a user are modest in comparison to others. Perhaps that's why I've never had a problem with the camera? It will likely continue to serve me well without issue for the rest of my life.
With all that said, if I were tasked with adding another Leica M body to the mix then the M5 would be my choice even though I don't care as much for match needle metering systems as some do. As with the M6 TTL, the design of the shutter speed dial found on the M5 would be the primary reason for this choice. (Plus I happen to really dig the looks of the M5.) My guess is that you'll be very happy with the M5. Good luck!
