I am indeed at Portsmouth! Are you thinking of going there then?
Erm, depends what you're after... The city centre halls (Guildhall) are (obviously) in close proximity to the uni buildings, but they can be really noisy at night and get a ton of fire alarms. They cost more to live in and they're self catered. Oh, and James Watson smells pretty disgusting. They do make going on nights out a LOT easier though because you're pretty much on top of all the clubs.
Langstone Student Village (I'm currently there) is 3 miles out from the uni campus (free bus service though). It's a bit cheaper to live out here (rent is less), and if you're in QEQM (which I am), you're part catered so get one meal a day cooked for you. The QEQM rooms have a lot more storage space, and nicer bathrooms - I have a proper shower instead of the wetroom that the other ensuite halls have. It's MUCH quieter out here, and I have never had a fire alarm as of yet (feel like I'm missing out on a fundamental halls experience though). Also, if you want to bring a car to uni you need to live here. Nights out from here are more difficult as the bus stops taking people back at about 00:15, so you need to get a taxi back which costs about £8 (but if you rally a bunch of Langstoners together it's not so bad).
The other halls are Rees & Burrell - they're by the Southsea seafront (about a 10-15 minute walk from the uni) and they're both fully catered. Apart from that I have no idea what they're like (except that Burrell looks terrible and Rees is stupidly expensive).
Best bet is to apply early because you're far more likely to get what you want... I left it a bit late and got put in Langstone, which wasn't my first choice but it is cheaper to live here and the bus service is fairly regular and free. I found that living further out means I am going out less due to the effort/extra cost of it, so I end up not wasting my money on booze!