55 gallon pony tank

[gobytron]

an overflow i an easy install.
No hardware involved so you can add one anytime without much change to your system.

With seahorses though, you have to be very careful with your intake/outtake as seahorses suck at a=swimming and are very easily sucked into anything that offers the chance.

A sump is convenient, and you're right gives a great place to hide equipment in addition to adding extra water to the system as a general buffer against error (consider in your 55 gallon with all the LR and LS you're really only looking at about 30 gallons of water capacity). I don't think they are necessary though by any means.

Like a lot of other things in this hobby, just another choice in what works for you and your needs.

and as you've already noted, a sump/overflow would be an easy add on later.