I'm guessing... or I guess.....
A: How about over there? That place looks like a restaurant.
B: Yeah, but with all those servers in white tuxes, I'm guessing it's going to be expensive.

"I'm guessing...." sounds natural to me. And I feel like saying like that . But I always use "I guess..." only.
Would you tell me the difference between "I'm guessing " and "I guess ",please?

Question

I'm guessing...  or I guess.....
A:  How about over there?  That place looks like a restaurant.
B:  Yeah, but with all those servers in white tuxes, I'm guessing it's going to be expensive.

"I'm guessing...." sounds natural to me.  And I feel like saying like that . But I always use "I guess..." only. 
Would you tell me the difference between "I'm guessing " and "I guess ",please?

Answer

To me - there's very little difference. Being a continuous tense, 'guessing' can suggest a little more thought is going into it, but generally they are interchangeable in this sort of situation.

Answer

These are usually interchangeable.  The only example I can think of where they are not interchangeable would be this:

"Are you going to the game tonight?"
"I guess (so).  There's nothing else to do."

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