Hi! I've stayed at both hotels you mention, they can´t be compared. The Best Western one is not that good and the Plaza is really nice but very expensive. Why don't you try Liabeny? It's a nice, clean, safe 4 stars hotel that's not too expensive and it's location is great. Just three or four blocks from Puerta del Sol and about two from Gran Vía. You can walk your way through Madrid from this hotel. The rates are about 14,000 pesetas in a single room and a bit more for a double.
I hope you find this information useful.