I bought the Energy C-50 speakers for my front and surround. The spec said it is only 85db (I somehow thought it is 87db), is that kind of low or sufficient for a 150 sq ft room? Should I get the Polk M10 (on sale) instead?

I have the C-50 and they definitely fill a room with sound, plenty loud enough to fill your space. You'll get better connectors on the C-50, the M10 only has plastic and the C-50 has metal.

I bought the Energy C-50 speakers for my front and surround. The spec said it is only 85db (I somehow thought it is 87db), is that kind of low or sufficient for a 150 sq ft room? Should I get the Polk M10 (on sale) instead?
If you've already bought them try them out and see for yourself.
The Polk m10 aren't all that efficient either - 89db vs 85db isn't that great a difference in terms of how much power they require.

I thought 89dba is 4 times more efficient than 85 dba?

The reason I don't want to try it first because returning a opened item has restocking fee.

I thought 89dba is 4 times more efficient than 85 dba?

fee.
Not quite. A speaker with 3db more efficiency will double the SPL level at given power level. So 4db difference in efficiency will only give you just over double the SPL.