The first decision is the type of game you like to play. Are you into skirmishes or full on battles. A good compromise is movement trays so you can do both.
As to shape, I use square/rectangular for troops that fight in close formation and round for massed troops. I like the regular vs irregular look.
There are also a number of manufacturers providing irregularly shaped mdf bases that take round individual figure bases. The best of both worlds. You can move large numbers of troops quickly, but can still remove casualties. (I hate casualty rings!
)
Next is the material of the bases. Metal is very good for magnetic movement/storage trays. Plastic is inexpensive and comes with the plastic troops. MDF has the advantage of the afore mentioned irregular movemet trays.
My first 25mm AZW collection was mounted on cardboard like you get on the back of a pad of paper and troops were stored by unit in foam lined plastic pencil boxes about 6" x 10".
Never broke a british soldier, but regularly lost Zulu shields and assegais.
I' m showing my age because now much of my collection is in foam lined carrying cases.
I hope this helped.