Hi Phil,
For one a glasshouse is much more expensive than a polytunnel. That is why as a hobby grower one usually only uses it for propagation. For two glasshouses stay warmer in winter, because the glass insulates better than the standard foil used on polytunnels. For three a glasshouse usally needs some kind of basement or groundwork underneath. This makes the construction more expensive and more time-consuming.
When using a glasshouse for indoor growing it could be that you have to plant a bit later than when using a polytunnel. It could also give you the possibility to overwinter or to grow plants with a higher need of warmth, but that depends a lot on where you are located and whether you heat the glasshouse in winter or not.