You will have to look at playing styles here - these two armies are on opposite ends of the scale. As you probably know already,
SM are elite, but every one of their units is strong when operating by itself.
Tac marines are some of the most flexible troop choices in the game, being tough, strong and dependable.
Guard on the other hand tend to have rather poor infantry choices. Not saying they are a no go, but Guard squads naturally are weaker compared to almost every other standard troop choice in the game. It's in tanks that the
IG come alive, and the Valkyrie of course. They undeniably have the strongest armour in the game, and in support with waves of Guardsman they overwhelm the enemy. As described in the codex, while space marines are like a sword of precision, the
IG "are akin to a mighty hammer that is cumbersome, yet sweeps all its foes aside"
something along the lines of that
so it depends on how you want to play it. Both codex's are still very much playable in this edition, it's just how you want to do it - the emperor's sword or the emperor's hammer?