I generally use sanding sealer for the 1st coat and varnish for the top coats. I buy mine at SWP but any paint store will have the equivalent. The reason I use sanding sealer is it dries faster and sands easier than polyurathane. Most polys aren't compatible with sanding sealer so I use varnish. Varnish doesn't dry as hard as poly but it allows me to do a nice acceptable job in the shortest/easist time frame.
That's not to say one coating is better than the other - they both have good points. It's up to you to choose which finish is best for you
