If you were looking to brew 10 gallons, you can go with a 15 gallon brew pot.
I brew in a 3 vessel 2 pump keggle system. I can do 5 or 10 gallon batches. I thought what
@sicktght311 said about running was well said, plus the mentions about keeping your taps going. I like to keep all four of my taps operating. I do have the flexibility to brew a big batch if needed, but since I like to brew different styles and my good friends don't live close by, I typically only brew 5 gallon batches, well I am aim for 6 actually, and with regular brewing I can keep them flowing. It's just that I have a few occasions a year where I like to bring my jockey box with 4 kegs that puts a crunch on the supply.
I put a bit of work into shining my keggles, so if I were to upgrade, I would
potentially go to a 15 gallon system, but I would definitely go the TC route. Not that the NPT is horrible but it would be influential in letting go of my keggles.
Last thing to think about though. What types of beers do you like to brew? If you are brewing 10 gallons, it's a larger fermenter or doubling up on your fermentation vessels. If you are using fermentation chambers, you need to think about what you have or you may want to look at glycol chillers. As an example, I have two mini fridge chambers. If I brew a 10 gallon lager, I have just tied them out for 3-4 weeks. I can't brew another lager any time soon. I think I am about 50/50 on ales and lagers, possibly a little higher. I am looking to build a glycol chiller though, but then I will need to adapt that to my fermentation vessels. I am not exactly familiar yet on what additional costs that will entail, Also, the temperature control issue is a reason I am still at the beginning planning stages of getting a conical.