Spare domain name... add as sub or host separately?

I have a spare domain for a decent low comp high volume local keyword relating to my moneysite.

Should I:

1. Just url forward to money site and build backlinks to it.

2. Build content on it to my moneysite and host as a sub domain of moneysite

3. Build content on it to my moneysite and host separately?

Just interested in views on the best way forward.

Cheers