The bare-minimum that I'd buy should have a 50 Newton.
The guys I've known who do Surfing, Water-skiing, Kayaking, Water-sailing, Canoeing and all sorts of crazy water sports stuff, would go for a 50N rated aid.
It also depends on how much are you going to bank on that, right? And, mainly this is applicable to all those who knows swimming or rather most suitable for competent swimmers. So it does imply that the help is really at the hand and within reach without much of a hassle.
The 100 Newton thingy is more suitable for usage in more troubled waters, probably for ones with yachts and Powerboats in waters where the help is at a relatively longer distance. It should be used by Non-swimmers, or may be 'Not-so-good' kind of swimmers in calm waters since they assure a reasonable safety from drowning in relatively calm waters.
Its rather a 'Do-whatever-you-want-and-get-away' kind of a safety aid. Its suitable for Swimmers as well Non-swimmers in troubled water. Absolute safety, rather as @nivag said, its more of a inflatable Life-Jacket and indeed gives reasonable assurance of safety from drowning.