From experience and what I know of the GK player he will either bring his Strike Squad spam, or his Terminators, and for the space marine player is normally (at this point level) 1 tactical squad with Rhino, a 5 man Assault Squad, a Librarian, and Scouts I think, so Plasma will help a lot.
I am thinking of running the Helios suits as Plasma only, that's why I asked, but the Fire-Knives have always done the job I set them for, Jump out of cover and destroy a transport, the jump back, next turn jump forward and Rapid Fire them to death, it has work every time.
I was thinking SMS but the meta where I am is mainly SM type armies, so Plasma works better, and I do have 3 Broadsides (Missileside configs) and 3 Railtides, so yeah, I just prefer the Railtide at this level as he is my long range meltagun of sorts, and to make a 10 man Tactical squad foot slog the entire board (if I get first turn) is just too good to give up.
But in essence, Im stacking up on all the AP 1 and AP 2 guns I can so I will beat the opponents squads/models/units on the first or second volley, without worrying about Armour Saves, know what I mean? Causes I've had 12 Fire Warriors fire at a SM squad and only 2 Tactical's die, but my triple Plasma always kills at least 4, guaranteed (Im good with my dice) but yeah