Well, they're identical on the table unless you get into running named characters. So it's really a question of whether you like painting dark blue or yellow more. The Crimson Fists never really seemed that interesting to me-- they've got a slight Spanish flavor, but otherwise, they're your sort of cut and dry Codex Chapter. They happen to have some setbacks in their recent history, which makes them more interesting than the Ultramarines, but that's about it. If you do mess with named characters, Pedro Kantor sort of steers the army in the direction of spamming Sternguard units since they gain quite a bit if he's around. It starts to be kind of gimmicky, and spamming units is boring and arguably a problem with the hobby as it currently stands.
The Imperial Fists are sort of in a similar boat, but for some reason, I've always found the prodigal sons of Dorn to be far more interesting than their second founding counterparts. Their fortress-monestary orbits Terra, and they send fleets all over the Imperium. Bright colors look good on Space Marines, and even if that wasn't the case, the Imperial Fists are a First Founding Chapter and the modelling possibilities are even broader than what you'd get with any other Chapter. The Imperial Fists (and any other First Founding Chapter, for that matter) have artifacts of their Primarch, ancient suits of armor, more suits of Terminator armor than other chapters, etc. The Imperial Fists aren't played as commonly as the Crimson Fists, to boot, since there isn't a gimmick you can do with a named character (or at least, not as good of one) from the Imperial Fists. Lysander is also best used in conjunction with a Sternguard squad, but he doesn't seem to dictate the composition of the army by his special rules the way Pedro Kantor seems to.