I have both and I purchased the mini ruler foot from Farrell ( a bit pricy at $55) If you are quilting on a domestic machine with a low shank foot, you will probably need the mini, as the most popular rulers are the larger ones, and if you have a small throat space on your machine, the larger sized rulers would be a problem for you to move around a small harp size. Also, you will have to be very careful using the Farrell mini rulers as they are on the fragil side, and don't have much in the way of markings. But, the one I like the most is this one
http://stores.cscountrystitchin.com/mini-pe-swirls/. It makes a pretty design, after a little practice. But the quality for these thin rulers is not there.
Since I purchased a new machine with a high shank , I can use the thicker 1/4" rulers now, which I much prefer using to the 1/8" rulers. You have more to grab on to to keep it in place and there is not as much chance of hoping the ruler. If you are limited to getting the thinner rulers for a low shank, then I would suggest the Westalee rulers.
When using curved degigns, I really like the interior rulers. Meaning you quilt inside an enclosed area. Much easier than trying to go around the outside. You can still use the thicker 1/4" rulers, but you are restricted in using the back of your ruler foot with a low shank. As an example, this is a ruler I really enjoy using :
https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1
It's 3 pieces, so you can quilt in the interior or the exterior, giving you 5 sizes, but using the interior is much easier. You can also use the curve for other designs as well (not just flowers)