I'm actually in the pre-purchase learning stage myself, but I'm afflicted/blessed with some OCD so I have pestered zillions of people and learned all KINDS of stuff in the last few weeks. I will share what I know so far and maybe someone with more experience can elaborate.
I personally have been looking at Kromskis and I am just about set on the Minstrel, but one of the limitations of the Minstrel, as well as many other of the widely available brands is an orifice of only 3/8". From what I understand, the orifice, right off the bat, limits you in spinning and/or plying thicker yarns as they must pass through the orifice. Majacraft has a "delta" orifice, and if I'm not mistaken, that means anything goes, while Louet comes standard with a 5/8", and many manufacturers, including Lendrum and Ashford, have bulky/plying accessories you can add on to accommodate.
As for the ratios, it is about the twist, and typically the smaller the number, the thicker the yarn. I believe a 2-5 is recommended for bulkies, whereas laces and other fine fibers can require from 25-40. Again, many manufacturers offer add-ons for lace, as with bulky.
For me, since I'm on a budget, I am just looking to get the most bang for my buck with something I like the appearance of. Lendrum and Louet have wheels that may or may not be more versatile in the similar price range as Kromski, but I don't particularly care for their appearance and dislike the required accessory buying, where as the Kromskis come with lots of stuff.
The Woolery has some wonderful information here
Someone here gave me this page, which was also informative: here
There is a page with statistics on wheels here although I'm not certain how updated
I have been advised by a few that the Lendrum folding is about as versatile as you can get and in a nice price range too. They were originally for bulkier yarns, but now range from lace to bulky.
Hope this helps!