Green gem has a very tight compact rounded habit. It is feasible to use these as a hedge, they take to trimming well. I have them in both forms - as stand alone specimens and as a continuous hedge, and both do equally well.

Green velvet grows larger for me, and has a nice mounded form to it without any leggy base - it keeps the mounded form all the way to the ground - I have one on the northeast corner of my house as a foundation planting and it's just beautiful. My 6-7 year specimen is maybe 4-5 feet wide at the base, but again boxwood are very forgiving and respond well to trimming to the size you'd like to maintain.

I try not to trim past mid June for my zone, to give it time to harden off for the winter.

In my opinion, I'd use green gem either as a continuous hedge or as a onesey in a smaller space. I think the green velvets have a softer form to them.

For foundation plantings, and depending on how much space you have, you might want to consider green velvet as they grow larger and will fill in nicely in time, with a nice mounded form rather than a roundish ball habit that the green gems have (though equally nice, just a matter of personal taste).

Just remember to clean out any fallen leaves from nearby trees from the inner part of the plant in the spring. Too much debris in the interior can cause the plant to lose foliage.

Also, cover them in the winter with pine sprigs or wrap in burlap - they'll appreciate the protection from winter sun.

Give a nice deep watering mid November before it goes dormant for the winter. Come spring you'll be rewarded with beautiful boxwood.

More info than you asked...hopefully this helps.