Depends, the Vapor-X runs quite a lot cooler and has more overclocking potential in the sense it will not reach temperature limits as quickly as the regular versions. The HD 5850 isn't really even that hot to be honest and the reference design is decent enough - with the HD 5850 I don't think it's worth it, but if you plan to overclock then I suppose it is.
it is worth it on some cards like the 4870 but a card like the 5850 there is no need, use the money saved to either save up for another one in crossfire, or the new cars at the end of the year, which will cope with the dx11 games much better
If you're basing this strictly on temperatures then maybe (the idle temps between the two cards are very close but the load temps differ by around 10-15c), but for performance not at all. Like Lmeow said the hd 5850 cards do not get very hot. Anyways, I believe both Sapphire cards do not have the ability to adjust voltages, so if you plan on overclocking I would suggest getting another manufacture.
I myself have a 5850 vapor-x and an Asus 5850 directcu and the latter has lower idle & load temps with way more overclocking headroom. It manages to oc higher on stock voltages and I still have the ability to over volt if desired. I was running them in a crossfire before but decided to put them in separate systems because I did not need the performance.
i had the vapor-x 5770 and it never went higher than 68C running FurMark. that said, my 5850 now at stock voltage with 860/1160 doesnt go over 72C in FurMark with the fan at 55% which is audible, but i always play music so i can't hear it. i also think the reference design of the 5850's look cooler than the vapor-x model, so if aesthetics is an issue there's always that to consider
I don't find it to be worth it as like iTouchedYou said, the ref. 5850 runs cool on the stock design. Although the Vapor-X is quieter and cooler a bit, there is no need for it because I use a reference design 5850 running 950/1205 1.16 and at 50% fan speed, it is audible, but if you have headphones or headsets on for gaming or music, you won't hear the fan at all... So just get the reference design, save money and CF.