On a paid stay, I would go with the Park Plaza Soi 18. As mentioned above, it is competitively priced with local competitors, and a comfortable hotel.
On straight points, the Radisson Blu at 38K is better value than the others at 28K.
As for the Park Plaza Sukhumvit, its gold upgrade is better than the Soi 18's as it only has 2 room categories, and it is a little closer to transportation hubs. I preferred the staff and atmosphere at the PP Soi 18 though. Would stay there for a ~THB2-300 cost differential, and if having a view is a priority (basically no view at the PP S18 and Radisson Suites). My deluxe corner room in the southern pointy end of the building had a nearly 270deg. view ranging from the Exchange Tower to the Sheraton!
Gold member gifts are a little pleather elephant and a branded mug
The Radisson Suites is at the end of a featureless side street, a negative unless the proximity of the Soi 11 nightlife is desired. Their larger rooms are substantially bigger than at the Park Plazas', but upgrade policy haphazard. I get the feeling this hotel isn't as well managed as the others.
Room decor and quality at the PP and Radisson Suites are very comparable. Small detail, but the harsh PP shampoo just doesn't agree with my hair; however it is very effective as laundry detergent
The Radisson Blu is a class or two above, and very new too boot (although the others are very well maintained).