I'd go for the 17". You can't use the 12" on 12.5" blocks to square up....

I have the 12 inch, and keep forgetting to use it when the occasion arises. Mind you, I haven't touched a quilt for nearly a year now, busy making purses and totes. However, I plan to get back into it by mid November, after the craft show I am participating in. Very handy for cutting small pieces like HST etc

I have both sizes. I seem to use the larger the most.

I've got the 12". It's all the store had when I bought mine. I don't think it rotates nicely so I don't rotate it at all anymore. It is the one that is next to my machine for paper piecing when I just need a quick slice.
Having said that... It isn't wearing as nicely as my big Fiskars mat. It doesn't appear to be very self healing and is looking really funky. It doesn't get a lot of use but it still looks terrible considering its age. I wish they could have used a better material in making it...

I have the 12" and rarely use it. I like the Fiscar 7" and use it every day.

I had the 12" and then bought the 17" on sale for $50 at Connecting Threads. Haven'r used it yet because I have been unmotivated to sew for more than a month. Am looking forward to using it though.

I have the 12". Wish I had bought the 17" instead.

I have both and I use both...so don't really hav an opinion of which I like better.

I have the 12in. and really love it. Of course, the bigger one might be better. Barb

I have the round Brooklyn Revolver and love it for smaller blocks. I also have the 17" Olfa for the bigger blocks (and it also worked well for cutting out the Spicy Spiral Table runner)