You are basing whether or not you should upgrade on the numbers and letters used in the name but you should be basing it on the fearures and whether or not you need them, in my opinion. When HS3 does come out, my guess is that cubase will include “HSSE3” anyway so it will be the same dilemma. HS2 has a vastly greater library than HSSE2 but, more importantly, it has much greater capability to edit the programs, for example the Flex Phraser, modulation, envelopes, effects, etc. HALion is even more powerful in these respects. There are some demos you can try to see if the features benefit you enough to warrant the price.
Hope this helps.