I think, first and foremost, a variable voltage mod should be the priority. this will allow you to adjust the device to your liking and find the combination that works best for you. In addition, you will find that you no longer will notice the performance deteriorating over the charge of a battery. VV mods maintain the same voltage all the way through. The Omega and Prodigy will not offer that for you. I own both the ProVari and Joker.
The Joker is less expensive to get started with (119 for device/battery and charge kit, + 17 for voltage reader) than the provari (159 for device, additional 40 for batteries and charger, additional 20 if you want the 18650 batteries). approx $140 vs $200/$220
The Provari does offer longer battery life than the Joker. I am a heavy vapor so the results are specific to me but is all relative. The Provari with 18500 battery lasts me 7-10 hours, the 18650 lasts me 12 -16 hours. The Joker batteries last me 4-6 hours. As long as you have the backup batteries charged, like any other e-cig, it is just a matter of how often you have to change the batteries.
The Joker is considerable smaller in width than the Provari. The Provari is much heavier than the Joker, by a wide margin. As far as appearance, I think they are both attractive, the Joker comes in many colors, Provari 1.
As far as function, they are both similar in what they do, the Provari does have a built in voltmeter and ohm reader for the atomizer resistance and the voltage is changed though clicks of the ignition button. The Joker requires a multimeter or voltage meter (listed in the pricing)and a screwdriver to change the voltage. From my experience, once you find a voltage you like, you rarely change it. I don't think I have changed voltage in 6-8 months, once I found my preferred settings.
They both perform the same, they apply a set amount of voltage to the atomizer and it is consistent. The performance itself is identical.
There is a big difference in warranties, the Provari is 1 year while the Joker is 90 days. I have sent my Provari back twice now and they returned it promptly and were very pleasant. I only had one repair with the Joker and I did have to pay for it ($36 repair + shipping). Both companies were great to work with and when I had issues, they called me to discuss.
I love both products and use both, I find the Joker easier to carry around but the Provari is a endurance beast (18650 batteries, that is).
If you are very cost conscious, the Joker may make a better value. The only repair I have had was from a long fall on concrete. The 2 Provari repairs were for a programming issue and a malfunction.
I think the big difference is in size. Some may say that the Provari has more built in features, which I agree with, but in the end the features (built in voltage voltage adjuster and battery/resistance meter) are convenient to have but I personally do not access them very often so I dont find them critical to the enjoyment of the product.
There ya have it, a comparison of the 2 leading (In my opinion) variable voltage devices, through my experience.
Good luck.