It depends, both hotels are nice, but I'd give the edge to the Budapest Hilton, on the Buda side. Where you stay really depends on what you want to do in the city, and how you will be arriving. If you are arriving by air, the Budapest Hilton wins hands down because you can get a free shuttle from the airport to the front of the Hilton and vice-versa. If you are arriving by rail, the Westend Hilton is about half a block from the rail station.
The view from the BH is much better than the WEH, and I prefered the rooms at the BH. Both have good exec lounges, but the edge definitely goes to the WEH. There are more things to see/do on the Pest side and with the WEH so close to the train/subway, getting around is quite easy there. You really can't go wrong with either hotel, both are good. Here's a thread that can give you even more info to help you decide.
http://www.flyertalk.com/forum/showt...light=budapest
Have a great trip.