Thumb hole stock are very comfortable if they are made to fit your hand ,the A5 is also comfortable if its made to fit your hand. I would suggest getting a good laminated stock in the A5 pattern
We all know where to get those , right kids :p
The thumbhole isin't gonna flex any more than an regular desgined stock maybe even less due to its desgine. I personaly like the A5 style and getting one in laminate makes it better casue you can sand it to fit your hand perfectly , I have to do this to all of my to make my fat paw happy. Drop Joel Russo a line and ask if you can see some of his thumbhole stock and some of his A5-L's , hes a great guy to deal with and offers great products at better than reaonable prices.
Here is a pic of an A5-L that I did for my 260 , and I have another in the works for a F-class gun