I think the extra 1/2" is the seam allowance for 2 1/4" seam allowance.

I learned a long time ago whichever ruler you start with you should use throughout the whole process. I have noticed markings on one brand are often off by a "line" or hair. I also often forget about it.

When I want to square up a block, sometimes it is very important to put the 1 in. and 1 in. corner on the corner of the block. Especially if there is a diagonal seam which also needs to be squared up. I couldn't work with a ruler with the seam allowance on all sides. It would drive me nuts.

I have a couple smaller rulers that have the 1/2" on 2 sides and I can not tell you how many times I have cut my fabric wrong (I do a lot of my sewing late at night and often when I should be in bed!). I finally took the worst culprit (a 6 x 6 1/2) and put it with my crafting items so I use it with wood and foamcore not with fabric! For squaring up, I usually go to a larger square so I have a better overview of the block I am cutting (a 12" ruler for a 10 1/2" block).

All I've ever used is the 1/2 inch one (and as far as I can recall all my rulers are 1/2 inch extra on one side). Can't say I've ever seen a 1/4 inch on both sides ones.

I like the 1/2 inch on them and can't imagine using one that has 1/4 inch on both sides honestly. But I do think it is whatever you are used to using and/or comfortable with.

Thanks for the heads up to watch how rulers are marked if you are buying a new one!