First of you need to calculate Day 1 power requirements, and then future power requirements. So if your current Rack takes 30Amps at 120V now, but you might need 60Amps in the future, your Load would be around 7.2KW (this is a simplification as it isn't taking Power Factor into consideration). You will normally want to spec that to be around 80% of the Peak load of the generators.
As for Natural Gas vs Diesel. Do you current have a source of natural gas, or would you be using Tanks? What is the availability of natural gas in the case of a long outage? IS there a possibility that your city/municipality can shut off the gas feed? the pro is that if you have a feed, you don't need to fill the tank. The con is that if they do shut off the feed your are pretty much SOL (hard to go to store and get Nat Gas Tank in a pinch)..
IN the case of diesel, you will need to continue to fill the tank ever X hours (Where X is determined by the size of the tank). With Diesel, in the case of an extended outage that is going to go past your run tank, you need to make sure that your supplier is able to pump (since the pumps require power and and if your out for a long period of time, they might be as well).
Also you would potentially want to look into an ATS (Auto Transfer Switch), so that the generator will start automatically and switch over to it without you having to manually do anything. In this case you will only need say 10 seconds or so of UPS power, so charging the batter back up won't be too bad.
You haven't mentioned your cooling. If you lose power, and but continue to run the rack on generator, what about cooling the room. As teh room gets hotter the fans in the equipment will run faster, and draw more power. Doesn't help to have everything powered up if is just going to melt down in the case of an extended outage.