Depends on lots of things
On a fractional rig, the main is in fact a bigger sail, but it is a less efficient sail (no matter what type of rig) than a jib/genoa. So one shouldn't discount the effect of the jib.A new jib will likely improve pointing ability and performance whereas a new main would have a significantly lesser effect on pointing ability.There are many problems with worn sails, but a common problem is excessive stretch under strong wind conditions. In fact an "old" sail may be too powerful in strong wind conditions. This is where you want your new sail to be flatter (and twisted).I sail a 24' boat with a fractional bendy rig. I can flatten a worn out main more easily that a worn out genoa by having the lowers and jumpers adjusted appropriatly and by adjusting my backstay as often as my mainsheet when sailing in windy conditions. It's very hard to flatten out a baggy stretched genoa.In fact, I faced the extact same decision for this boat a few years ago, and chose a genoa first, and then a main the next year.It depends on lots of things, we race and pointing well is critical, but that may not be as important for you. A jib/genoa experiences much more abuse with each tack than a main, and they tend to wear out fast too!I suggest you talk to your local sailmaker about the type of boat and the type of sailing you do, and they will probably have the best advice.Chris