Some advantages of Titan are:
- abundant nitrogen,
- a thick atmosphere that will protect against radiation (the extra distance from the Sun also helps)
- Lower gravity (and atmosphere) makes access to and from nearby space easier
- Thicker atmosphere and lower gravity makes aircraft and helicopters an easy and effective way to move people and cargo around Titan
- More things in relatively nearby space to explore/mine
- Easy to cool machinery etc to the atmosphere
- Possible subsurface ocean -- not quite sure what this is good for, but its interesting
Some advantages of Mars are:
- Abundant iron and other heavy elements
- Usable solar power
- Gravity closer to Earth's (may be relevant for health, we don't know yet)
- Much warmer -- and less heat loss to the thinner atmosphere
- Three to six months from Earth using about 4-5 km/s of delta-V (from LEO) versus 3-5 years to Saturn system and about twice the delta-V
- Radio takes a few minutes instead of an hour or so.
Both have abundant water ice it seems, and carbon (CO2 on Mars, methane on Titan). Mars seems like a more plausible short-term prospect due mainly to its nearness, but Titan may ultimately be the more valuable long-term option due to its nitrogen and its radiation protection.