Yes both of them get super big i wouldnt suggest anything below a 150 for them they need alot of swimming room and even alot of room to turn. you figure then dont swimm backward so they need to turn in your show(depth wise) tank.
I would say, 75, just for the fact that deeper is harder to light, just like a lot of other people have already said. Not sure if I will ever upgrade from my 40g, guess I need to get it full of corals before I have to worry about stuff like that :P
go for the gusto man a 120 wont break the bank!!!! i see them on craigslist list all the time for like 150$ here in vegas... them tangs will love ya if ya do...well at least for a minute then you need a 240 to 300 and they should be okay...;)
i will take the other side - keep the tank you have now and learn the hobby. gain from this experience all you can and start to save for that dream tank
75's, 90's, 120's are all nice tanks but maybe one day you'll wish you had gotten a custom made cube = better to wait and make the right choice for you
this hobby is expensive enough withouot wasting money by not taking time to plan out a system that will take you into the future and not just another one step upgrade followed by another
i would suggest getting a tank with deep dimensions. Somthing shallower around 20 inches easier to get par to the bottom and then get something thats pretty wide. I paid 500 for my 100 48 24 20 and wish i would have went wider. Good luck hopefully whatever you decide you end up happy with it.