Both names are nice. We have a friend named Lincoln and we call him "Linc", which is a cool nickname. Lincoln sounds strong and righteous. Spencer sounds fun and intelligent. (Don't know why, but that's what I get from it.)
I agree with the other moms who say to wait until you see your son. You might be surprised and come up with a completely different name! Jake is AWESOME. That was going to be my son's name but then I had 4 girls. I thought about naming one of them Jake anyway but I didn't want her to end up in therapy!! ;-)
I also agree about keeping the name(s) secret from your family and friends until it's official. I had an awesome name picked out for one of the girls -- wanted to use the name since I was a little girl and gave this name to all my dolls -- but then someone gave me such a hard time that I didn't use it. I still have some regret over that.
Finally, maybe I overthink things, but I always tried to think of how kids can be picked on by other kids because of their names. If they have funny initials, if their name rhymes with or looks like something weird, etc. You know how cruel kids can be. Something did pop into my head with one of the names but I don't want to even say it and put any ideas in your head (unlike a lot of people who did that do me!)
Just go with your gut. You have time to keep "sleeping on it". I know you're asking for everyone's opinion here, but if you know which name you prefer, don't let anyone talk you out of it. You picked beautiful names for your girls so I'm sure you'll be perfectly happy with whatever you name your son!!