That would give a storm raven a 4+ invul, and let the tech marine make repairs while he's embarked
The PFG does not work while inside a transport. Its in the
DA FAQ.
In a Storm Raven you pretty much have to skies of fury them. I've rarely seen terrain that can completely block a storm raven, so if its visible, and it has a unit of Centurions inside, you are asking for trouble. If you leave them inside the raven, you are asking your opponent to shoot it down. It carries your Dread and your Assault Cents. I know I would fire pretty much everything in my list at it to blow it up. Each Centurion would take a S 10 no armour save allowed hit if they were still inside and it crashed. That would be enough to instantly kill the entire squad. Its far too enticing for an opponent to not take it especially when your list (Snorri) lacks any real firepower outside of that raven.
If you Skies them, then your opponent will get 1 free round of shooting up close and personal, as your Cents will pretty much have to be in their face to do anything in subsequent turns. Furthermore, when you take into account that almost every army (excepting a
CC army) takes an Aegis, your Assault Cents lose their ability to strike at I against the prime targets that will no doubt be behind said Aegis. The
LRC can give you the assault grenades that the Cents so desperately need. If you have them with Ironclad launchers, they've lost their firepower.
I think the land raider is a better choice for a transport than a storm raven, but I do not feel Assault Cents are a good choice. You pretty much have to play as White Scars if your intention is to move your
LRC filled with Centurions across the field. You need the extra 12" from the scout move. Your largest unit will now not benefit from chapter tactics, and you essentially have 1 turn to survive to get into assault. This is at least a more preferable option to running them as another chapter, and hoping for first turn so you can get to midfield before your
LRC gets blown up.
Against a pure
CC army like a Green Tide, Assault Centurions can bring enough tools to at least serve a purpose. I haven't seen an Ork army in a tournament yet this year though. If its a
TAC list, Assault Centurions are a points sink. I don't see enough horde armies (even in friendlies) to justify spending the points on a unit that is only better than a
TH/
SS squad against hordes.
On foot, I'd go with melta guns, if the main threat is smash, 6 twin-linked meltaguns is a pretty serious deterrent. Might as well stick in a marine character as well (3+ invul eternal warrior character is a nice choice).
Where do you get the 6 Twin linked meltas from? Each assault cent is only armed with either 1 twin linked flamer or 1 twin linked melta. In a unit of 4, you would have 4 at max twin linked melta. Unless of course this unit is 6 Assault Cents on foot with a character in the unit. Assault Centurions on foot are probably the worst choice in the entire codex. You've essentially paid a crap ton of points for a unit that acts as a deterrent. Without a transport you are never going to see assault unless the opponent wants to. You've put over 1/4 of your points (2k) into a unit that will essentially act as a weapons platform for 6 Hurricane Bolters.