They look remarkably similar rather one much better than the other. The exh cam gear might fatten it up a bit earlier but last time I checked Trent said it makes no difference on an RB25.
The 2835ProS costs more but comes with everything to bolt on exactly to the stock location (from dump, new intake, nuts, bolts etc) vs a turbo requiring some fabrication. Given the threads out with SAU'ers having issues with fitting Hypergear turbos it goes to show that some 'simple' fabrication to some is a lot more complicated and expensive for others. The guys that have had issues for several weeks/months probably wish the spent on the HKS unit (if they were still available) those that have had no problems are laughing all the way to the bank.