There is very little difference in wot,but a big difference in planning power.I have ran tons of 15 and 20hp 4stroke outboards in the last yr and the only major difference is quite a bit more power with the 20hp if you have more weight in the boat that the 15hp struggles with getting up on plane with.At wot it's so minimal you wouldn't even notice unless you had a gps and even then it's a mile an hr to 2 tops.Yeah you gain 5hp in reality but it's very deceiving.I have ran hundreds of outboards so i know my stuff,plus i buy and sell outboards for a living(small outboards up to 25)so i test all of them before i sell them.
If you went with a 25,imo depending on what boat your putting it on,a 25hp 4stroke is extremely heavy compared to a 20hp.Not worth the time and energy especially if you have to take it off and put it back on a boat.
Now a 25hp 2stroke compared to a 20hp 4stroke is a bit different.I will tell you i have a 20hp 4stroke suzuki and a 15hp 2stroke merc and hands down the suzuki is in a whole different class.It's all of 97lbs and has just as much power as any 2stroke i have ever ran due to the efi.I love merc,tohatsu,yamaha or even honda but the suzuki 20hp is so far ahead of the class it's not even close.
I love my 1995 15hp 2stroke merc,it sounds like a racing engine but the suzuki uses near no fuel,is as clean as it can get,quiet as heck and the best for the money hands down.
Sorry didn't mean to go off subject,if you have more weight then usual go with the 20hp for the extra emmmmphhhh,if it's just you or 2 of you the 15 will do just fine.To me the price difference is worth the upgrade just because of the power difference alone getting on plane especially if your near a ton of rocks and oysters like i am and need to get on plane in a hurry,if you don't have any objects around you and your not in a hurry go with the 15hp.Anymore help let me know.