The systems are very close but they are different. It's similar to D&D and Advanced D&D. Except there seems to be less difference between RMC and RMSS/RM2 than there is in D&D and AD&D. Harp is also very similar. And I believe HARP is a simplified version of RMSS. And I think RMFRP is the first simplified version of RMSS which became HARP. Not sure on that though
But basically they are so close you could play any of them from your ruleset with very little adjustment.
I got to try RMC and RMSS before choosing one. We were playing Advanced D&D but one player and the GM wanted us to try RM. The player wanted to play RMC and the GM wanted RMSS. We tried RMC first and then RMSS. Our group loved RMSS and played as a group for about 20 years. The GM introduced another GM and we had the luxury of 2 different but equally good GM's. They took turns doing campaigns and that was fantastic.
From my limited experience the main difference is RMC doesn't cap the development skills. Thus if you get 3 points for each point of development at 1st level you get the same 3 points at level 100. So 100 levels of development would be 300 + your bonuses. RM Standard System/RM 2 has diminishing returns on skills. Say 3 points for the first 5 times you buy the skill. Then 2 for the next 5 times you by it. And then 1 for the next 5. Then .5 for the next 5 times you buy that same skill. So training that skill 100 times in RMSS is not really worth it. RMC might have different racial and stat bonuses, but once that is figured in initially you won't really notice that.
I don't know if this is true, but RMSS seems to limit players power more. Of course talents and such can greatly change both games. So even the term limit is relative. And that is really a choice for your GM and group to make. But to our group the RMC system seemed more exploitable. Our group of 6 players had at least 3 min/maxers. The RMC fan was our worst offender
. So we decided RMSS was the best option for us. We loved it and haven't looked back since.
In theory this encourages you to be more well rounded and develop other skills. Hopefully the roleplaying ones. Or skills that are important to your class. Like a cleric developing more religious skills. That way when you say the dungeon is East so we head East. The GM can have you check your direction sense skill and everyone should have it. And one person in your group should know which way is east. It sounds small but that can add a ton of depth to your games. Or in the case of our cleric he can develop his religious lore skills more and have a chance of recognizing that holy symbol on some guy we just met. And instantly knowing that something is wrong.
Of course in RMC or SS (2) house rules and customization are very much a part of the game. So a good RMC GM can easily run a good RMSS style game. And a good RMSS GM can do the same. And as you know RM is so flexible you can play Medieval, Space, Western or whatever you want easily.
I have both rulesets. Ironically I bought the RMC rules from the guy who wanted to play RMC so badly. Then I slowly got all the RMSS books that I wanted. Which ended up being just about all of them. RMSS seems to have a lot more options to add to your game. Though you can probably buy a ruleset from either game and easily apply it.
Some say RMC has less core books. I guess RMC had one book with all the rules. However, the RMC I got came in a boxed set and is 3 small but separate books. Ironically my RMSS book was one book that had the basic rules and spells in one book. Then I bought other books for new rules and options I wanted for RMSS. Though my one RMSS basic rules is bigger than all my RMC books combined. LOL.
Though RMC has some Lord of the Rings settings. And those are the best RM setting books I have seen. I have a book on the Dwarven cities for RMC that is my favorite. Though underground races for RMSS has the most interesting source information I have ever read. And have led to some of my most classic characters.
So in the end, since you haven't played in a while. The differences are so small, you probably won't notice it at first. And by the time you remember the differences you should be level 5 or higher in your new campaign