Halogen bulb confusion for Chromega

[ac12]

My Omega D5 color head bulb spec is for 250w 24v halogen bulb, and an Omega part number.

I've seen references to ELC and ELC-E bulb for this enlarger.
But even in the ELC family there are many variants ELC-3, ELC-5, ELC-7, etc.

I am also confused as some of the bulbs are rated at 50 hours, and others at 500 hours.
Is the 50 hours many on/off cycles, vs 500 hours more continuous burning?
All things being equal (which they probably are not) I would go with a 500 hour bulb over a 50 hour bulb.

So what do I go with?

thanks