2 Companies, liked 2nd Company, 1st Company offered me a position...?

I went in for 2 interviews Monday and Tuesday of last week. I like the 2nd company better, but the 1st offered me the job on Friday. I asked them if I could take a couple of days to think about it (the pay is really low). I'm supposed to give them my answer by today (Monday). The 2nd company said they would get back to me by Wednesday of this week. Can I email the 2nd company asking them if I am a good candidate for the position, letting them know I have been offered a position elsewhere? Is that unprofessional? This is URGENT! Need opinions ASAP! :D