I can't comment on what frame to get, but this talk of Aluminum being fragile is just not true.

What matters more is the weight of the frame/tubing diameter/quality of construction rather than whether it is steel or Aluminum. They can both be good if built heavy enough.

Steel has a fatigue limit true , but track bikes seem to be used hard (you are not spinning low gears in the saddle like you do with a road bike) . Most of the time you may be exceeding this fatigue limit if you are pounding a big gear out of the saddle depending on how heavy you are and how heavy the steel frame is.

There is a fatigue test call the Efbe test which applies a very high "out of the saddle" load to a road frame for 100,000 cycles. Steel does not do well in these tests for whatever reason. Aluminum does great though. Sure aluminum doesn't have a fatigue limit, but it seems to have a better strength to weight ratio.