Selecting tank for upgrade

[CarterJ]

I have a 20 gallon tank currently, and am looking to upgrade in the next couple months. I am trying to decide between 2 different tanks; a 12o gallon tank that is 5' long, or a 125 gallon tank that is 6' long and but about 3 inches shorter in height. We would like to get a tang or a pair of tangs (yellow tang and kole tang, also open to suggestions on this decision). I like the looks of the 120 gallon tank better, I like the look with the 3 extra inches of height, and I like the cabinet set up on the stand better. However, I wonder if the tangs would do better in the 125g tank with an extra foot of lateral swimming space, or if it would really make much of a difference.
Second question - I currently have a 10 gallon sump and will probably upgrade to 15 or 20g (depending on what will fit in the stand cabinets). I have about 20 or 25 pounds of live rock in the current tank (just guessing) and I am currently curing 50 pounds of dry rock to add into the larger tank. I have a protein skimmer that I built: 4' of 4" pvc pipe. There is going to be a good sale on canister filters at a store near by on black friday (they will be half off). Is the canister something I need or that I should invest in?
Thanks in advance for any advice

 

[Oscar47f]

In my opinion you should choose the tank based on what you think would look best in terms of aquascape and how you picture it would look best... Regarding the tangs there should be enough space in both tanks for them, i have a friend that keeps 4 tangs in an 80 gallon bow front (not saying i recommend doing that) but it works out very well for him and the fish are fat and happy, i would feel comfortable with two tangs in that 120-125 gallon tank and i personally like longer shallower tanks rather than taller tanks i think they're easier to work with and more space underneath for filtration... and now a days a lot of people get really creative with their live rocks and tend to do minimalist aquaecapes and thats ok not to have the recommended 1-2 pounds per gallon of rock as long as your filtration can manage ! so i would get a nice sized sump with a good skimmer and other equipment... and if you'd like to use the canister thats fine just i do hear a lot about them braking and causing nutrient issues if not cleaned properly and as redundancy i wouldn't make it the soul nutrient export but if it works then why not...