Like anything you're going to get your good reports and bad. I'm biased towards Merc, I've had 5 new ones over the last 20 years, only had issues with 1, and I put that down to the ability, or more the inability of the dealer in fault finding. On the odd occasion the overtemp alarm would go off and they were unable to rectify, so I made a deal and traded it for a Suzuki. Both these were 40hp. That motor now would be 4 years old, and I'm now thinking of trading it back for a Merc, just not happy with it compared to the Merc. It run rougher, and is not as smooth using the tiller etc.
My other boat had a 2 stroke Merc which was 2000 vintage, a few years ago we upgraded that to a 115 ProXS only for fuel economy and the smoothness of a 4 stroke, it has been faultless, and love it.
We're in the process of looking at getting a new boat built that will be in that 6-6.5m mark and it will definitely be getting a ProXS on it.
In saying all that, you are going to get those that have held on to the old 'Black Anchor' mentality, it's a bit like the Holden/Ford battle, there are true believers on either side.
Interested to hear more about your new boat but, I love the 'research' and process of buying something like this new.