Which tank should i get?

90 gallon. a sump is a plus!

 

Welcome to 3reef! I would go with the 90. bigger tank = more fish, coral, cool stuff can be put in the tank. The sump is a plus. I would go with one whenever doing a reef. The skimmer should be upgraded though. The two fish are compatible, but with caution.

 

90 and buy a skimmer rated for at least 200g, if you need to work on the sump you can get a simple 20 or 30g tank and use that their very simple to build with some plate glass and silicone

I'd never re-use someone else's live sand and rock anyway, no telling what kind of bad stuff they have going so starting out with new isnt a bad thing, can buy some dead sand and base rock then seed it very cost effectively and no matter what anyone else says I'd probably never put a trigger and puffer together, 2 very predatory and territorial fish, the only way I'd ever risk it is probably in a 200+ gallon tank where they can both have their own areas and never butt heads, the first time the trigger gets annoyed by the puffer the puffer would probably end up dead

 

Oh one more thing, if you do change the sump & decide to build your own, be sure to go with a long tank, rather than a tall. This will allow for a larger footprint, meaning more space for equipment. Water depth shouldn't be all that high in the sump anyways. Just be sure to have high enough water level that your return pump won't go dry & be sure to have a space in the sump to hold the extra water if your DT overflows. For a large tank such as the 90 I would allow 6-9 gallons volume in the sump extra.

 

Both need a better skimmer.. And the 20g sump for a 90g tank isn't that good.. Get what you can afford.. Think about the expenses you will have on the long run..