
Not many to chose from but De Vere Grand Habour in Southampton is apparently Southamptons only 5* or if you cant decide on which city to go for try
Solent Hotel and Spa. It is slap bang in between the two locations and is a very nice place and just 2 mins off the M27 at junction 9 (used to live 2 mins drive the other way!!). It is in front of a lovely quaint outlet village, Whiteley.
Or the Hilton in Portsmouth. Choosing which city really depends on why you are going? If its for historic then obviously Portsmouth is the way to go (great huge outlet village at Gun Wharf Quay) or Southampton has a huge shopping centre (West Quay).