I think you might be confused as to some of the terminology...
An RO/DI (Reverse Osmosis/DeIonizing) filter isn't something you put on your tank. It hooks into your home's plumbing supply and is used to create 100% pure water to make your replacement salt water. I'd consider it an absolutely essential piece of hardware for reefkeeping, but it doesn't hook to your tank directly or filter any of the water in your tank. That said, if you don't already have an RO/DI filter, get one no matter what. You'll thank me later.
Canister filters aren't really a great fit with saltwater tanks. You can use them, but they require significantly more maintenance to keep them functional. Your best bet, especially if you want to turn your tank into a reef, is to start off by adding a sump to your tank, and putting a protein skimmer in with it.
Basically, there are three kinds of filtration at work in most reef tanks: Biological, mechanical and chemical. Biological filtration is by far the most important, and that's largely handled by your live rock plus any other media (Marine Pure blocks, for example) that you've added to your system. Mechanical filtration covers things like filter socks, filter floss, strainers, etc.; basically, it's using some kind of media to physically trap contaminants in your water. Finally, chemical filtration uses chemical reactions to bind and trap various harmful compounds out of your water. Activated carbon is an example of chemical filtration.
A protein skimmer doesn't really fall neatly into any of the categories, but it handles elements from all three. You don't have to have a skimmer, but they are probably the most versatile and powerful single filtration element after your live rock. So, all that said...
If you were going to just do one thing, I'd get an RO/DI filter to make sure that your water quality is the best that it can be to start off with. That'll improve the quality of your water changes, and if you do nothing else, then stepping up your water change schedule can compensate for some holes in your filtration system.
If you do two things, get the RO/DI system and a HOB protein skimmer. If you don't want to add a sump, you can skip it for now, but you will definitely want one when you upgrade to your 120. If you really want to use a canister, you can, but be aware that you'll likely need to be doing maintenance on it at least 1-2 times a week.
