What's your opinion & why on water pump replacement parts?

[BearLakeFishGuy]

So, its been about 5 years since I changed the impeller on my 150hp Yamaha.  I started looking around and now I have a few questions and wanted your opinIons.  No one will be right or wrong on this post.  
1)  Do you just change out the impeller or do you replace the whole enchilada (O-rings, housing, impeller, case, bolts) with an entire kit since you're already in there?
2)  Do you spend the extra money and buy factory (Yamaha, Merc, OMC, etc.) parts or do you buy OEM replacements (Sierra, Mallory, etc.)?  The factory parts are between 2-3 times the cost of the aftermarket OEM replacements.  THANKS!!