I've been to the glacier area multiple times. Franz Joseph is the best regarding accommodations/places to eat keeping in mind Franz Joseph is pretty minimal EVEN ON A NZ BASIS. Fox does have Lake Matheson; however, as someone has said, the two are only about a 40 min drive apart. I'd reco Franz Joseph.

... but all the ohhs ahhs are so weather dependent. If you get 4 days of bad weather regardless of which destination you pick (which is common in the rain forest), you'll be ready to off yourself. But if the weather is good, your helicopter landing or glacier climbing tour will be GLORIOUS.

The drive from Franz Joseph to Fox to see Lake Matheson will be disappointing if the weather is bad (clouded in). But just like the glacier tours ... with good weather, Lake Matheson is GLOROIUS !

BTW, you can do a walk up to the glacier face of either on your own ... no tour is needed.

What time of the year ?