I have a Yashicamat 124 and a Mamiya 645 Pro, so I have the camera you have (well, almost - your G has gold plated contacts for flash, more black and less chrome, and different internal gears, but they work the same and the lens is the same) and one you're considering.
The things to ask yourself are what you don't like about the TLR and what you hope to gain from going to 645. From what you say, the TLR is going to suit your style (street shooting with WLF) much better than the 645. The Yashica is much smaller and handier, easier to carry around. The 645 Pro and Pro-TL, but not the earlier 645 and 1000s models, offers mid roll film changes with film backs. All of then offer the option of exposure automation with the AE prism finder and film advance with the winder grip (I have both) and interchangeable lenses. Do you need or really want these things? And do you need or want them badly enough to give up the square format, the quiet operation, and the size and weight of the Yashicamat? I don't personally consider a WLF very viable for a 645 camera because I mix a lot of landscape and portrait shooting, but that's just me. Oh yes - the inserts for the 645 are easier and quicker to load than the Yashicamat and you can carry several pre-loaded making film loading even faster in the field. Combined with the 15 shots per roll this might matter if you tend to shoot a lot.
I actually enjoy shooting more with the Yashicamat and its simplicity. The Mamiya is far more versatile and much like shooting with an overgrown automated 35mm SLR.