Syruss,
Some quick answers to your questions --
Steel tanks come in HP and LP flavors
higher pressure = potential issues with getting a complete fill. This is mainly an issue with HP steels. Some folks say that the higher pressure will put more stress on your regs...others say that this is not an issue with the high quality regs out there today and with regular reg service. HPs use DIN valves -- should not be a problem for most dive shops to fill. The main issue is that if you go HP, in most cases you will need a DIN first stage. If you plan on switching back and forth from yoke to DIN -- you will need an adaptor. This can be an issue if you travel alot or rent tanks.
Gauges -- yes, assuming the same volume of air in the tank and equal consumption rates, the rate at which the "needle" moves down will be different. If you are used to dealing with 3000 PSI at the start of your dives all the time, starting with 2,600 is no different -- just may take a slightly different calc. to come up with your turnaround pressure.
More data -- HP tanks tend to be lighter than LPs of the same capacity (weight on dry land). HP tanks also tend to be more negative in terms of buoyancy.
I've just scratched the surface here -- for a more complete discussion on steel vs. AL, HP vs. LP -- try these links:
Start here:
http://www.scubaboard.com/t3488/s.html
Then try this:
http://www.scubaboard.com/t2886/s.html
Then the mother of all scubaboard tank threads:
http://www.scubaboard.com/t905/s.html
For more info -- try this cylinder FAQ over on scubadiving.com -- has a useful section on steel vs. AL:
http://www.scubadiving.com/members/faq.php#S9
Sorry to drop all these links on you, but this is one of those topics you need to do some reading on to help zero in on what type/size tank will work best for you. You will get many opinions, but to make a good decision, some research is necessary.
Enjoy!
