492 or 493 ? Which is better ? Or is the only difference a preference of no antenna , or antenna ?

Pretty much just the antenna

I prefer the shallow 493i receivers over the 492i antenna-less ones. I only use the 492i if I am hurting for foot-print space (like on my 12th scale cars).

Another Sanwa Rx question, since we are on the subject... What am I missing on the RX-49T vs. a RX493i? The 49T is half the price.

I've asked that question before, never got an answer. I haven't noticed a difference between them so far.

In going to guess the frequency. The rx-49t uses 2.4 GHz FHSSS , and the rx-493 uses 2.4 GHz FHSS5.

It seems like Sanwa comes up with some new alpha/ numeral to make the previous gen of transmitters not work with the new receivers.

That's simply a mis-print. An "S" and a "5" look similar and Sanwa has no technology called FHSSS. They are both FHSS5 or more simply FH5 protocol receivers.