It's more likely its the composition of what you're eating and drinking, rather than the brand.

Experiment when training as to what upsets you or doesn't.
Personally i find after a certain point that "real food" is better than gels, bars, energy drinks etc.

Last 100 mile TT i did, i filled 2 of my 3 bottles with nuun electrolyte tablets. My body didn't like it in 80 odd degree heat. I ended up grabbing a bottle of plain old water from a spectator (thanks!!) , which is what i've generally used before (with just a gel at each bottle swap), and that calmed my guts down.
Lessons learned :)

If you live in the States, I work at Skratch Labs and could send you some samples to try. Just shoot an email to info@skratchlabs.com ATTN: Joy slowtwitch

Personally I take the view that there's not much to choose between the brands, and I go with whatever the on-course nutrition is. Figure the benefit of not having to plan and carry all my nutrition outweighs any benefit that one brand offers over another.

The key is getting the mix of solids, liquids and concentrations right. And that varies a lot depending on distance, weather and individual. On a hot day I need a lot more fluid than on a cool day, so either I need a more diluted concentration, or I need to drink bottles of straight water alongside the energy drink. On hot days I also find I have less tolerance for solids so prefer to get all my calories from liquid. Distance impacts me as well. 70.3 I don't need a lot of extra calories to get through the day, so can play it pretty safe and get through on 200-250/hour which is unlikely to cause GI issues. For IM I need to push the envelope and get the calories in at a higher rate, which leaves less room for error if I don't get the right amount of hydration in. I don't run well if I've taken solids within ~2 hours of running, so on 70.3 I tend not to eat anything solid. But IM is too long a day to go liquid only, so I'll use the first half of the bike to get some solids in.

I think the key things to figure out and practice are:
- Your calorie burn rate, which you can estimate reasonably accurately from a HRM/power meter that knows your zones, weight, etc
- Expected finish time for your distance/fitness
- Above 2 factors will enable you to estimate how many calories you need to take on during the race. As a starter for 10 assume you have 1500 cals of glycogen stores when you start the race, and you get 50% of your calories from fat. E.g. if you're burning 800 cals/hour and your expected finish time is 6 hours, then you need 4800 calories to get through the race, of which 2400 will come from fat and 1500 will come from pre-existing glycogen stores, leaving you 900 calories to take on. That's 150 cals/hour. There's some estimates in there and you don't want to finish with the tank completely empty, so up that to 250 cals/hour and you should be in the right ballpark. Do your own calculation, then practice and refine as necessary
- On top of the above, you need to get a good feel for how your sweat rate varies in different conditions, as that determines how much fluid you need to take on along with those calories. You can weigh yourself before and after training, pee colour and how much fluid it takes to fully rehydrate after training will also inform this. The more you're sweating, the more you need to think about replacing lost minerals either via drinks with electrolytes and/or salt stick
- Everything after that is personal preference - whether you prefer gels, liquids, solids, what flavours, etc

Not quite - the 1500 glycogen cals you start off withare deducted from the carb total and not the overall total. So:

7600/2 = 3800
3800-1500 = 2300 cals of carbs that you need

Good point that you can't take on any calories while swimming, but if you're aiming for a 9.5 hour finish then swim time is going to be close to an hour, so you should still have >8 hours of time to take on nutrition, which works out at comfortably under 300 cals/hour.

Plus those are all estimates - burn rate varies by individual, as does fat burning rate, as does the amount of calories you can take on per hour, as does the amount of glycogen your body stores. And all are somewhat trainable. So take whatever data points you can (cal reading from your power meter/HRM, etc) to get you in the right ballpark, then practice and refine from there and be ready to adjust on the day if things aren't panning out as planned