Stalker bolter can be very useful if you want to stay mobile. The lack of ap can be mitigated by the one warlord trait (forget the name).
I believe you mean the Assault Bolter here. Stalker Bolter is Heavy 1 with better AP.
Outside of Deathwatch, I'm not sure how much I would lean on the assault bolters. Deathwatch have the special issue ammunition, which can compensate for the loss of range and AP compared to the standard bolt rifles.
With regular Marines, you're gaining 1 shot in the 15-24" range, but losing 1 AP at 0-15" and losing 6" of maximum range with the assault bolter. If you are confident you can keep your enemy more than 15" away, then they're a good choice over the standard bolt rifle. But I'm not totally convinced the trade-offs are worth it. (But I'm a Deathwatch player, so my experience using the normal Marine codex is limited.)
The stalker bolter I'd recommend this for objective holders. Maybe a 10 man squad you can split.
I'm not personally sure I would ever recommend the Stalker Bolter -- except on Captains, where it's either stalker or assault. Yes, the extra range and AP are useful, but so is the ability to move and fire without penalty, and doubling your shots at shorter ranges.
If you want to use Stalker Bolters, they're definitely going to be a backfield unit, parked on an objective. But I see that sort of a unit as a supporting element to the core of Intercessors armed with bolt rifles.
For the most part, the standard bolt rifle is most universally useful. It has great range which is always nice.
I would agree with this. It's a good all-around choice. For a first unit I'd go for these, play a game or two, and decide which (if any) of the specialist bolters you feel would fit best with your play style.