I think it would depend on how big your GH is and it's intended use. I have a small 6x8 HFGH & as the seasons change so dose my GH set up.

I use container to maximize my space, but if i had a hugh GH it'd be cool to have a tropical in ground room!

This is a great question. I'm assembling a 6x8 greenhouse this weekend, and am also wondering about planting directly into this very rich plot of soil I just rototilled and dug up. I had anticipated installing the GH on top of the plot.

I had planned small concrete piers on the ends with treated wood to secure the GH. I'd love to be able to plant into the dirt directly, but I suspect there are good reasons not to do so...

I have raised beds, in-ground (ground level) beds, and containers. The ground level beds and the raised beds function, by and large, the same.

Containers:

*Portability (++++)
*Ability to alter the growing medium easily (+)
*Ability to treat safely with systemic pesticides (should the need arise) without affecting other plants (+)
*Ability to better inspect the plant and thereby identify pests(+++)
*Greater temperature instability (-)
*Greater susceptibility to over- and underwatering (----)

Inground beds:

Just the opposite...
*Greater temperature stability (+)
*Can't easily change your growing medium (-)
*Can't get underneath every leaf and around every plant to inspect for diseases or pests (---)
*Can't move them around (----)
*Can't use systemic pesticides without affecting nearby plants (-)

BUT...plants grown in these beds are much more stable, water-wise. (++++) They don't easily get root rot because there are no perched water tables...and you can more easily skip a watering day with no adverse effect noted in the plant.

I would add that the growth patterns are a little different, at least for perenials woodies. In ground beds and raised beds, woody plants act like they do when you plant them outside. They tend to sit there for a year sometimes, then after a year they'll take off. The same plant in a pot will take off and sometimes flame out.

With regards to tropical rooms, I think its quite common practice to use a mixture between beds and pots.

Where I built my tropical greenhouse, the soil was poor, not well drained and full of pests. For the beds, I dug out some tenches, lined them with membrane and filled them with a loam/organic-matter mix. Any large spreading plants like gingers and anything that doesn't have any specific requirements goes in the beds but everything else stays in a pot. As long as theres a lot of foliage you can barely see the pots and it ends up looking quite natural.

I think it would really depend on how and what you want to grow. It probably doesn't really matter what method you use but more about how well you can look after it.