I use the cheapest, and buy them in a batch. But I never bother for casual rides etc, I just get food down me - although if I am feeling weak at the end of a weak I might get an IsoGel down me before going home.
Currently I am using SIS PSP22 carb powder (plain, I mix my own apple barley with it), and IsoGel (forgot the brand Hi5), I have loads of samplers as well from wiggle and planetx. You can buy a High5 sampler for £5 online somewhere.
The IsoGels are thin and easy to get down, the Hi5 EnergyGel are a bit thick and hard to get down, but their Banana was LOVELY, like a banana yoghurt. I've also tried some Apple IsoStar stuff which was very hard to get down and sticky - ended up getting everywhere, and some "fat burn" tabs (free with my PX orders) that you dissolve in water that make the water fizzy, and tastes rancid!
Now I am TTing, I try to get a good meal down me throughout the day, and then 1 bottle of PSP22 and the IsoGel before hand - im trying not to get so anal into all this yet.
Get a good meal down you the day before - I think people do carb loading with big pasta dishes etc
How well they work, I don't know - im pretty sceptical so I don't dont instantly hark on about benefits, and I'm not so sure. Sticking with a pattern for my TT now, see how I go before I adjust it.