i had to replace my water heater a few months ago and did homework on them then. the gist of what i gathered:
the tankless units are more efficient, but they're also significantly more expensive to buy/install... so it might take several years to break even and reap any of those cost savings. the energy savings directly correlate with how often it has to make hot water. if you have a large home with lots of users, it won't be nearly as efficient as you'd think.
so instead a better question to ask yourself is:
do you find yourself running out of hot water with a tanked water heater? if no, buy another tank. if yes it may make sense to go tankless...especially if your existing water heater is in a cramped space and you can't go bigger.
in my case i just opted to get another tank.
edit: oh, you might want to poke around on your gas providers website. sometimes they provide rebates/kickbacks if you get something energy efficient (tanked or tankless)