We all work hard on our army lists, even the fluffers do so (they just suck worse or more). The question often comes down to, what works best, assault or shooting. Do we work towards a harmonious balance of the two?, Or do we strive to perfect one or the other? I suppose in order to decide our direction we have to do a little analysis.
First off, lets not take what army we play into consideration. Lets look at army list building from what and how each type works overall.
Assault Armies. For these to work, you need to get in, hit hard, and dominate assaults. It is often easier to table your opponents with this type of army, as multiple assaults pulled off well kill squads much faster than shooting them to death. Win the assault (especially against multiple uits) and wipe them out with a sweeping advance. A shooting army normally cant remove this many models in a single turn, leaving assault armies to be the most efficient killing machines. Dont forget that shooting armies might have 3-6 rounds of shooting, while an assault army gets a turn every player turn, effectively almost doubling the number of rounds you are removing your opponents models off the board. Getting there is the problem. Once solved, charge forth and destroy.
Shooting armies. Technically on the table top, these can be the easiest of the two to play, although not generally to win against decent opponents. Your job is to hit the enemy so hard and quick, that they are on their heels the rest of the game. Alpha strike lists, or leaf blower type lists seem to be all the rage, although durable resilient shooting armies are starting to appear. Target priorities are the critical here. You need to master it, or that unit or two you were not dealing with well will destroy you. You only have a few rounds to do this. By rounds 4+ if you are not mopping up, you are losing or struggling. If you have mastered this army type, you will have many games only last an hour or so. These are often the quickest games, not counting deepstrike list types.
Now we come to the hardest of army compositions. Armies that balance out both, assault and shooting. Marines do these the best. Here you have to balance both and make both work effectively and cohesively. This army type is probably the most balanced with the least amount of weakness's. You dont have to overcompensate your army list here, as you can do both, or at least have specialty units that can do one or the other. This area is probably were most people fall. Its the most giving style of play.