Go for a new skimmer. You can always get a new calcium reactor down the road. They are more of a convienance, where a skimmer is more of a necessity IMHO.
First, what are you keeping? Second, what size aquarium?
How are you maintaining calcium now?
I'd upgrade the skimmer only if you feel it is working poorly.
FWIW, I had a berlin and replaced the rio 2500 with a sen 700 and the thing rocked. I was emptying silty scum 2-3 times per week. I hear good things about modifying the venturi as well. I think Kent makes a mod.
A calcium reactor is a great addition that won't go to waste. Why not mod the skimmer and have the best of both worlds.
Go for the Reactor... Skimmers are for Amateurs :lol: ... I actully had the Red Sea and it never really worked for me and then I weaned off of it and havent had another skimmer on there for 5 years.