Go on JM website and you can see that he offers different power levels on some of the most popular springers.He built up a custom spring for me and we had great communication,I really appreciate that.I do not know if you can still talk to him,I know you can talk to one of his employees and they can help you....
Knowing what I know I can not see how a Vortek system can be more powerful than a JM power spring.....I say this because my Vortek kit has its spring inside a covering or tube,it is an old kit though.As was stated both are very good.
Another thing is get the kit,not just the spring..the kit has better seals and comes with the "tar" that quiets the spring down...the Vortek kit has its spring captivated inside a tube,it is also quiet......
Another thing many of the air gun smiths use JM springs for their tuning work.....
Another example R-7 with JM kit,really liked it ,smooth,then put a Vortek kit in it,at first I didn't like it as much,it fired different,then I got used to it,it went poof instead of the old springer thumb....it is a good kit,just different...BTW my old R-7 had well over 100,000 shots in before replacing its spring ,it was new in 1981....good spring suppliers are getting harder and harder to come by,you have to have a large order to get them to make springs to your specifications.....