Soba, actually Shoba is Indian and don't make anything. They buy from other manufacturers and factor the goods under the Shoba name.
The results can be varied depending on what batch you are buying from.
I have seen good and also seen crap.
Vertex OTOH are Taiwanese and market their own goods, usually of a better quality but the standard of rotary tables is dropping by design.
At one time the worm was supported in the housing by either bushes or bearing with some form of thrust race.
Many nowadays are just steel worm straight into steel or rarely a cast housing and no bearings.
As John says, nowadays RT's are all much of a muchness when it comes to quality in the lower price bracket.
So the only way, unless you have loadsamoney or can pick up a good used quality one, is just buy the largest one that will fit your table comfortably. Remember though, when you add a chuck, they do grow in height, and a few of the smaller millers are restricted on clearance between the quill and chuck.
Another thing is, if you can afford it at the same time, try to buy a dedicated chuck to match the RT. It will usually save a lot of heartache trying to make backplates and location slugs. 6" and above in size will usually come with a morse taper location in the centre of the table, and you can get adaptors that allow you to fit Myford or Boxford(Atlas) screw backed chucks. Straight off the lathe, onto the RT. I think 4" and below only come with a plain machined spigot hole. Although the Vertex 4" does come with a 2MT centre.
The advantage of the slightly more expensive Vertex 4" & 6" models, is that when you get a little more cash, you can buy a cheap (around £30) and easy to fit dividing head setup for it.
I have used a 6" Vertex one for many years, and have never had cause for complaint.
Do a Search on Chronos web site for rotary tables and all will be revealed.
By searching around other suppliers, you can usually get the bits and pieces cheaper. But do take into account the postal charges from each supplier. Some say free, but charge a higher price, some charge a lot less for the bits but their postal rates are extortionate. Balance it out.