The answer to this question is fairly simple: User preference! I bet that was not the answer you were looking for, maybe a little more technical instead. Right? OK, lets look at these puppies from two sides: The first one might say: I want hydrophilic because:  - I do not need accelerator to kick it off (meaning starting the reaction)  - The urethane will not be so easy flushed out of a joint with a high volume leak.  - and the seal stays flexible. On the other hand the second one might say: I want hydrophobic because:  - with the use of accelerator, I can precisely time the reaction. - with the max amount of accelerator, the grout will react fast and will not be flushed out of a joint.  - I have a variety of choices going from rigid to flexible urethanes. "Are you still confused? Lets add some more arguments. Hydrophobicers (just made that up myself; is a person that loves hydrophobic urethanes) might say: "All the hydrophilic urethanes are TDI based." Response: Not quiet true. With careful formulation one can prepare MDI based hydrophilic urethanes. Look at our Sealant M, 100% genuine MDI based, not even a single trace of TDI. Or "All hydrophilic urethanes have the tendency to shrink when they dry out." Response: Absolutely true. The more water they absorb, the more they will shrink. The keyword here was "dry out". When will the cured material dry out? Only when the moisture source is removed. I mean that if the cured material is in a 100 % humid environment, the hydrophilic urethane will not shrink. Remember, the urethanes are used to seal "leaking" joints. In most cases, by sealing a joint, you do not remove the water source, you just redirect the source. The water will still be there in contact with the seal. A few exceptions are for example:  - Water treatment tanks where the water level can fluctuate a lot and expose the seal to "dry out". Or  - When you try to seal a below grade structure where the water table fluctuate a lot. Example might be Texas during June-September (is it obvious that I wrote this article during the Texas heat?). More arguments might be:  - I like the smell of hydrophilic urethanes better or  - Hydrophobic urethanes have a more professional look. We have tried to make our grouts smell like Channel No 5, but I am sure many grouters would be in trouble when they got back home. The point I try to make here: When to use hydrophilic or hydrophobic: user preference.