[quote=@Frengo]
EDIT: So for me it's going to either be a drop regiment, or a standard infantry regiment, I guess. Depends on what fits the RP better- I mean, I'm thinking we'll need to take down planetary guns and establish footholds before the Imperium can start deploying its regulars enmasse?
[/quote]

Both have genuine value here. Drop regiments are Light Infantry, and operate in a similar way to paratroopers. They can establish a foothold, or gain critical land before the ensuing invasion. As light troops, they're also useful as scouts and special forces. Regulars come in a lot of varieties, but I'll assume you just mean your typical lasrifle wielding foot soldier. If that's the case, there are three kinda you can go from there.

Line Infantry, which are just your vanilla generic guardsmen. They do guardsmen things.

Mechanized Infantry, which ride into battle, typically on Chimera. They also may have a few tanks.

Trench Infantry, which are equipped for trench warfare and sieges. So more or less static.

All of them have legitimate value here. The best way to end this is with maneuver, which favors Drop troops and Mechanized forces. Line Infantry draw a balance, and Tench Infantry are focused toward static fighting.

If we're not careful(This will probably happen at some point even if we are), it's exceedingly easy to fall into really long sieges of both planets and cities. It becomes hard for even Astartes backed by Imperial Guard to seize a city with dozens of millions of people living in it when who you're fighting is committing total war. Maneuver is critical, but being prepared for a slog is also important. Basically, there's no really wrong choice here. We need everything we can get at this point, and no answer is particularly better than another since not many roles have been solidly filled yet.