Building a bench power supply

Hi All

I am building a dual supply for general hobby work - 5v fixed and 0 to
15v variable. Suit microchips, TTL stuff, and small motors etc etc

Basically 2 separate supplies in the one box, with the monitoring meter
switchable between V and I, and between supply A and Supply B

No sweat with that.

What I am not sure about is whether to have the supplies fully floating
or earthed or floating with a removable link to earth or a removable
link for the negs.

ie both neg output terminals earthed? Both just joined - ie common negs
-

The variable supply will be based on a switch mode power pack I happen
to have - output fed to an adj voltage regulator - hassles?

The other on a xformer power pack feeding a 7805 etc

Housing is a plastic box - can fit with a 3 pin lead/plug if I need the
earth

Will appreciate views, ideas, and advice

David

.