I’ve used SiS before and find they taste OK, but are heavy as they are a no water gel. Gu is my current favourite and they are a lighter with water gel, so carrying four at a race is OK – although they take a bit of ‘chewing’. I use non-caffeine (lime flavour) at miles 5, 10 and 15, then save a caffeine gel (chocolate flavour) for mile 20. Always have water soon afterwards. (I’ve tried ‘torq’, but find they unsettle my natural methane balance!)
Best bet is practice on a long run (10 mile plus would be OK if it is just a fuel/ unsettling of stomach experiment), if you have time.
(Can't comment on zipvit, but hope the above helps).