If you have just a single day to travel and enjoy a park, I think Thorpe would be the better option of the two. Whilst it is possible to get from London to Alton Towers by public transport, you would probably spend the bulk of your time traveling. Thorpe Park has the benefit of being practically next door to Heathrow.
You've picked out the two best suited UK parks. When you return in a few years, you may also want to consider some European parks that are fairly simple to get to from the UK, including Phantasialand in Germany and Efteling in the Netherlands. There is also a direct train from London to Disneyland Paris.
Given you say Thorpe Park is on a 10pm closing on your chosen day, that will mean the park's Halloween events will have started. So in addition to the rides, from 3pm there will be a number of houses (they'll be called mazes) that are included with your ticket. Unlike many US Halloween events, the actors in the UK parks are allowed to touch you, so be aware of that!
If choosing Thorpe, I would recommend purchasing your ticket online from the park beforehand, so you have a scannable ticket either on your phone or printed out. The queues for ticketing can be quite long and inefficient, so this would mean you would just go through security and then the turnstiles.