I have used the megapot (8 gallon), spike kettle (15 gallon), and blichmann (20 gallon). I prefer the blichmann and have been using it for almost two years now. For me the horizontal configuration on the spike kettle was problematic, it caused the thermometer to fog during the boil (when using a bayou classic sp-10). I just ended up plugging that port. The packaging was minimal and the kettle arrived with the bottom dented. The Megapot was nice and more durable than the other two. I do think the boil off rate would be higher on the 20 gallon megapot vs a 20 gallon blichmann. The blichmann dimensions are perfect, and it includes a very nice dip tube, adjustable angle thermometer, and three piece valve. I know you said you don't need a sight glass, but the blichmann sight glass is useful, durable, accurate, and well engineered. I like having a slight glass, its easy to see when to stop the sparge and my volumes are always spot on.
If you are looking at a spike kettle, consider building your own with the same kettle or here is another version.
Also don't forget to check out these guys.