I am looking for recommendations for a 40 or 50 gallon natural gas water heater. Ours is at least 16 years old, and has recently started making noises like there are marbles rolling around inside. Also, we have noticed a gradual, yet significant, reduction in the amount of hot water we get.
I've investigated going tankless, but it really won't work in our situation. We live in a rowhouse and would have a very hard time finding any place we could vent the unit according to code. Additionally, I just don't want to fork over the cash upfront at this time.
I've found there are a ton of water heaters out there, and it can be hard to compare them. Are any of them relatively more efficient than others? Are there brands to stay away from? I'm hesitant to go with a powered vent unit, simply for fear the fan would die and I've never notice.
Any advice greatly appreciated!

All brands of hot water heaters are typcially the same. They use the same type of gas valve and pressure valve.
I have used the Kenmore brand in my home now for several years. Had good luck with them and price is fair.
Be sure to purchase the best they have for longest life. Also if you have hard water, I would suggest a water softener.
What the diffrence between the heaters are is the anoid rod that is placed inside. This is seen by the large bolt head on the top of the water heater. This is connected to a zink rod that corriods before the inside of the hot water does. The warranty of the hot water heater is dependent on the size of this rod. The larger it is the longer the warranty on the heater. And of course the more it costs.
Be sure to get into the habit of draining off a little hot water out of the valve located on the bottom of the heater. This flushes out any silt which causes the noise your hearing. I would not suggest however to do this on the heater you now have if you never did this before. As quite often the silt displaces the water and causes hot spots that tend to leak once the silt is drained away.
As far a power vent, not needed unless the hot water heater flue pipe is oversized for the unit or you want to side vent the heater.
The tankless units can be traditionaly vented and would connect directly onto the wall next to the current hot water heater. If this wall does not exist, then simply constructe one out of wood 2x4 and plywood.
As far as the power vent failing, You will know as the hot water will shut off.