Here is what I own....
I own the 900mm (36"ish) rule with hook stop. I am contemplating the 300mm one too, sometimes I need one that is a little shorter. It is excellent. Doesn't flex, consistent, very well made. Probably more play than their t squares to have a perpendicular line drawn, but I don't use it for that. That plus the hook stop, plus the 2" rule stop are significant parts of my amateur work flow. I use the rule stop as a setup for my parallel guides as well.
I own the 1281 square: It is probably my most used for things like checking square on assembled parts, and when I need to do some quick and easy perpendicular lines. It isn't the best to use for measuring and marking (the woodworking rulers are tapered which really helps for the marking side of things.
I own the 18" precision triangle. I use it some, but not as much as the other one time tool I'll describe below. I thought about the 12" and decided the 18" would be better. Besides you can't draw an 18" line with a 12" ruler, but you can draw a 12" line with an 18" ruler. It is useful when setting up your MFT table, or making sure your saw track is square to the edge. Longer lines than the 1281, but still a bit large. If I had to do over, I might consider the
Precision Triangle or even the Fancier more versatile
MTR-18 Triangle from [member=59331]TSO Products[/member] instead. It just adds more features when combined with the MFT table. The TSO ones also are more readily available and will reach your workshop sooner. (this is not me knocking Woodpecker, I really like everything I have purchased from them, but the OTT take a while to find their way to your door).
I also own the 26" precision carpenters square. This is a monster, and it is wonderful. Great for marking lines on sheet goods, setting up saw tracks and your mFT table. It won't fit inside of cabinets to check for square assembly, though. But it has it's place and it ain't leaving my tool cabinet any time soon.
The only thing a triangle gives you the 1281 doesn't is the ability to draw that 45* line.