I shoot both 6x6 (Mamiya C330 TLR) and two different SLR systems with "rectangular" formats (Mamiya 645 Pro and Mamiya RB67).

The best reason to consider an SLR is to permit you to do the things you cannot do as easily with a TLR - close-focus work, work that requires precise framing, work that benefits from TTl metering, work that benefits from interchangeable backs, work that benefits from a motor drive, etc.

You might also prefer the ergonomics of one over the other - that is a good reason to switch.

Don't discount the importance of being restricted to landscape orientation before you have tried it - it just takes one picture per roll that demands portrait orientation to end up causing you great frustration.

I don't know whether this applies to the OP, but I have observed that people who tend to view their photographs on a computer monitor tend to use portrait orientation much less frequently than those who prefer to work with prints or project slides.