You're asking a number of questions here, so let's try to answer each in turn.
What weapon should be used against Marines? Well...obviously the 'best' weapon for killing Marines is the Dissie - it will ignore their armor and has a good strength to punch holes in them. However, does that make it really the best? And what qualifies as 'best' really?
(The answer is kitted out Incubi in assault...but I'm pretty sure you're asking about shooting, so let's stick to that
Space Marines are built to be tough - it is their job to (hopefully) not die easy. They do this with above average toughness and above average armor saves. To oppose that you want to either shoot them with weapons that ignore those benefits, or weapons that circumvent them.
A Dissie ignores their benefits (ignores armor, has a high strength so can wound easily)
A Splinter Cannon circumvents (ignores toughness, and shoots a lot to hopefully force armor save failures)
There are other options too, a dark lance, lacking vehicles to shoot, is pretty darn good at blasting apart marines, albeit slowly. We also have a number of missiles that are quite good at killing Marines. The reality is, most of those weapons are better used elsewhere, or are difficult to properly bring to bear in the numbers needed to do Marine killing.
Okay, so really our question is about Dissies and S.Cannons, the two primary forms of DE shooting.
Generally we bring our shooting via our transports, Troop selections, and Heavy slots.
Let's first consider the most likely platforms
===================================================================Ravager w. 3 Dissies - Cost 105-115
9 shots - 6 hits - 4 wounds - 4 dead Marines (2-2.6 in cover)
Value: Around 28 points per Marine kill (around 57 in cover)Raider w. a Dissie - Cost 60-70
3 shots - 2 hits - 1.33 dead Marines ( 0.66 - 0.88 in cover)
Value: Around 52 points per dead Marine (around 79 in cover)Venom w. 2 S.Cannons - Cost 65
12 shots - 8 hits - 4 wounds - 1.33 dead Marines in or out of cover.
Value: Around 48 points per dead Marine
So, what we can see is that a Dissie Raider is of little help (though it does do better versus Termies out of cover, and it does become better versus targets out of cover, point wise, if you don't buy it any upgrades) compared to a Venom - indeed a Venom is a better value killing machine of Marines pretty much all around. The Dissie Ravager is better yet, but not to a shocking amount, indeed, on average and if the Marine uses cover, the Venom competes *very* well versus the value of the Ravager on a point by point basis.
So, what this says is, Venoms are indeed excellent tools to kill marines - just expect to be killing only around one Marine each time you shoot a Venom at them. As long as you understand that's about what you should expect, you will understand the amount of Venoms you'll want available to do shooting.
If you have no need for lance Ravagers or fliers in your Heavy slots, a 3 Dissie Ravager is also a very solid deal, and will kill Marines quite well - specifically in the open, where it will chew them up.
Your second question is about Troops. Eh...personally I don't think our transports are death traps, and with 4 viable troop selections I thought we were doing really well in the grand scheme of Troop options.
Certainly if you want 'tougher' troops, than Wracks are the way to go, they are significantly harder to kill than our other Troop selections, which are all fragile in various ways. Personally, I find the fragility is part of the DE playstyle and choose to embrace it, I look to take units that inflict more damage quicker, and usually find Wracks to be a bit on the slow side in that manner, and thus I don't bother with them. I figure the best way to keep my units aliv is to just kill the hell out of the enemy units first. But, yes, if you want harder to kill Troops - take Wracks (or maybe Hellions with Baron, but that's a questionable thing and a rather different playstyle).