Imperial Guard armies, from what I've heard, tend to be taxing in the motivation department. Not only are there a lot of them, but a) you're going to be painting a lot of flesh and faces (unless you're doing
DKoK. In that case, enjoy working with resin

), b) there are so many little details that need to be gone over to make a model look complete, and c) you're probably going to have a TON of vehicles to paint, and depending on what tools you have at your disposal and your skillset could potentially be a huge pain (I know it is for me).
On the other hand,
SM are easy to paint, but hard to paint
well. So many of the different techniques like drybrushing and dipping that work well with detailed models (like the fatigues of the
IG) simply do not work as well on the smooth power armor of the marines. Prepare to do a lot of edge highlighting if you're going for tabletop quality, and get yourself a decent detail brush because painting some of the more detailed armor means being able to switch between smooth-armor-mode and little-tiny-detail-mode.
This is just my 2 cents, as my painting experience is pretty limited.