It won't make a difference if you train on an "analogue" aeroplane or not. All they care about is that you have the licence and ratings they require and the right number of hours. As far as only having one FTO on your CV is concerned, I can't see why that is important. I used 9 different training organisations / flying schools but it might be easier if you used the one. They probably won't really care anyway, and they will care even less where you did your hour building.
I reckon the best way to assess an FTO is to visit them, talk to current students and just take a look around. If the office is tidy, if they are in control of their process and if they seem organised then you will probably be OK. If the place is a mess and they don't seem to know who is doing what in which aeroplane and with whom, I would walk away. I have seen some very good and some very bad FTOs and you can tell within a few minutes what it is going to be like there.
Also if they can't be bothered to be nice to you at a marketing event, they won't give a stuff once they've got your money!