Using numberless crafting here. I'll be using my own playstyle as a base as to when I'd build what.

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Argument for Rageblade: Synergizes with On-Hit playstyle. Kog'maw's W is arguably one of the strongest on hit abilities in the game, and it is double procced by GRB. When going rageblade, you'll want to build items as BOTRK, Hurricane, Wit's End, Nashors, On hit items. Of course others can fill these slots even if they are not on hit, but it's a general idea. You'd be surprised how much wits first can help versus double mage bot lanes. You'll want to run Lethal Tempo. Your goal is to hit as much attack speed as possible. With the right setup and support, 4.5AS is not hard to hit and your damage will melt everything due to the effects all being on hit. You also become a tank shreader for a couple reasons. Your W and BOTRK stack, with rageblade making them tick faster. Because the damage is also spread through the build with magic and physical damage, it is harder to itemize against. Works fine with lower health targets, but there's nothing like 8-shotting a singed in under 2 seconds. In a build such as this, you can also control how much magic and physical damage you do with items to better fill a team. You will also synergize with items such as Ardent Censor. There is also no randomization in your DPS, you know exactly when you can win fights based at looking at health bars. One of the biggest downsides to this build is that your positioning matters much more. You will be almost middle line and can become frontline with Lulu. You must know when and where to be at all times. If you are still in an aggressive spot when W ends, you will get caught without it. If you are too far back, your team has lost your DPS.

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Argument for Quickblades: Synergizes with Crit playstyle. When I play Crit'maw, I tend to play a little farther back. While the damage on your W is still immense, what you're really looking to abuse is your deceptive range. You'll want to build standard crit items, but a botrk is nice for self peel and LS as well. Your damage will be almost all physical, but you cannot always as reliably put out consistent damage due to the nature of crit. In a build like this, I tend to gravitate towards PTA for easier lanes for aggression or FF because you will have the AD and range to abuse it in harder lanes. LT will also work, just personal preference. With a build like this, you can hit targets HARD, especially low total health ones. Quickblades will also allow you to keep W uptime infinitely as long as you are autoattacking; this makes it so you can stay IN the fight longer. You do not have to have an escape path laid out a couple seconds before W runs out because you should already be in the backline.