No the first one is popping whatever is on the stack at that location, the second one is considering whatever is at that position in the stack to be a memory address and you're saying whatever is at that address copy it into esi.
Consider stepping through the code through a debugger.

They look as though they should do the same thing. The only thing I can think of is if you have got an "assume ss" somewhere. Other than that, you could try posting a larger code fragment so that we can see it in context.

Are you preserving ebx, esi, edi ? These three registers are to be protected in function calls.

Your two code snippets do load the same value from memory though!

In Win32 The Stack Selector SS:, Data Selector DS: and Extra Selector ES: are typically set the same from the application point of view. In the older 16-bit Operating systems, the 'Segment Registers' as they were called then were almost always different !