I am starting my second hop garden this year. It will be a raised garden along my fence, facing south to get the sun.
The advantage of the raised garden is numerous.
Better temperature, easier to access, great drainage, non compacted soil, all things that will make hops even happier.
Over the years, your rhizome will grow and if you don't cut it down it could cross with other rhizomes as the crowd each other. What I'm doing is "subdividing" my raised garden. I am using cinder blocks (I've got about a billion) and I making containers that are roughly 4'*5'. There is no way the rhizome will grow through my blocks. I'm running the hop garden along the fence and in front of it I will have a raised garden for whatever non-hop stuff I plant this year.
I think a raised garden is a great way to control the area your hops occupy. Possibly more crucial than your soil is how you will support them. Put a lot of thought into your trellis. Try not to compact your hops (step on them) as you work the trellis. Ensure that your trellis can support both your hops AND a STRONG wind. Anyone can prop up a 20 foot tall peice of PVC. Will it stay up in a wind storm? I saw a guy that used an old steel fence. It was really cool looking, until it fell on his car during a rain storm. Not urban legend! I saw it! Your raised garden will be great, worry about your trellis.