In my opinion the survival suit is far more important than a dinghy if forced to choose, ideally I would have both, but that isn't always practical in a light aircraft of course.
How many people do you know who own a survival suite? Of those how many do you know that own enough survival suites for them and their passengers? Of those how many do you know that insist they and their passengers put their survival suites on?
Leaving that aside, the USAF amoung others would not agree with you.
Survival suites or dry suites are a great idea - but in reality most find them uncomfortable and most are unwilling to invest in 4 suites.
On the other hand once in a liferaft the raft will work better than a suite. You are out of the water and wind, so compared with a suite the heat loss may be comparable or less, and remaining afloat without inhaling too much water is not a risk.
Finally the average aircraft owner or group is likely to be prepared to invest in a raft or rent one when required but very unlike to invest in four suites.
I accept those who argue that getting in the raft is an issue. It is a great deal easier if the raft can be deployed before the aircraft sinks which is entirely possible. Landing in a big swell may result in loss of life in any event so in conditions where the occupants are likely to survive the landing the evidence would seem to suggest a good chance of the aircraft floating for at least a short while.
Take a look at the surface temperature data for the Channel for each month starting in January. The temperature in May is les than 3 degrees warmer than in January. Mid channel the difference is less still.
8.2
7.7
6.5
9.1
11.1
13.7
15.2
17.4
17.3
14.6
12.1
9.6
11.9
Life expectancy in sea temperatures of 10 degrees or less is one hour or less without protection.
The USAF advice remains:
Your best protection against the effects of cold water is to get into the life raft, stay dry, and insulate your body from the cold surface of the bottom of the raft. If these actions are not possible, wearing an antiexposure suit will extend your life expectancy considerably.