if you can swing it, go custom.
i find that the sound quality between the custom and non-custom earphones to be different. here's why.
when places like ultimate ears make a set of earphones for you, they take the ear impressions you send them and fit the earpiece exactly to your ear, but that's not all. they use sophisticated measurement devices to make sure that the audio response, when it reaches your ear canal, is perfect. that's not an adjustment you can make on a universal-fit earpiece, you'll always be losing something.
they trim and custom-drill the vents in the ear canals on custom earpieces to make sure everything is perfect when it reaches your ear canal.
now here's my opinion on custom 2-driver vs. universal 3-driver.
dual driver is superior in many ways to single-driver (at least when we're talking about balanced armature drivers; dynamic drivers generally sound best in single-driver configurations). it's easy with a 2-driver system: a high/mid driver, and another driver tuned for better low performance. this is really the only way that a balanced armature earpiece can deliver really high-impact bass, because of the limitations in frequency range in the drivers themselves, not to mention the physical limitations on the speaker coils.
triple driver design is actually a misconception on both the UE and shure products. what happens there is that there's dual low end drivers and the same single high/mid driver. on the shure e500, the dual low drivers are tuned to have more frontal volume and more low-mid coverage. on the UE-10/Triplefi 10, the low drivers sit back farther, and are actually lower volume and don't have hardly any low-mid coverage. i prefer this -- the bass is more recessed but WAY more detailed. you actually feel the soundstage of the low end.
the only company with a true 3-way IEM is westone, the ES-3. it's a true 3-way crossover, and this gives it a bump in the midrange that people using it for music playback don't care for.
if you want a very solid listening experience with bass and high/mids of the best quality, get the UE5c. it's the same components as the UE-5 Pro, just tuned differently and at a price point for consumers.