Unfortunately my EarSonics SM3s just wonked out after less than two years of careful use. I've narrowed down my next IEM to either the Westone Eilite Series or Ultimate Ears Pro models. My main use for these will be for drum performance and mixing purposes, so I prefer them to be as neutral as possible. Although I mainly play & listen to progressive styles of rock. My other main concern is which of the two is better durability wise... Any advice would be much obliged.
Elite Series or UE Pro models? You're talking about custom IEMs then?
Westone's CIEMs are all skewed warm; they're chiefly designed for stage use. If you mix with them, there's a likelihood your mix will be colder than normal. You can mix with them, but just remember to double-check on studio monitors to make sure your mix sounds right. Sound signature-wise, the ES5 is quite pleasant to listen to --- it's very organic feeling and envelopes you with a richness that is just easy to listen to. If you're coming from the SM3, then Westone monitors have a similar enough sound, but do keep in mind that neither Westone nor EarSonics designs sound signatures that go beyond stage use, and thus are all fairly warm and lush-sounding in the vocals, with somewhat relaxed treble. Nothing will come close to what is considered neutral.
Ultimate Ears has a range of CIEMs that range from quite warm (UE18) to quite neutral (Ultimate Ears Reference Monitors). Their recommendations for stage use are the UE18, but I personally would recommend the UE11. It's not as expansive sounding as the UE18, but the midrange is tonally more neutral-sounding, despite its sitting spatially farther back in the mix.
For mixing use, the ideal choice are the Reference Monitors (UERM). The UERM was a collaborative development with Capitol Studios to create a "mobile studio" --- they took the basic sound of the Yamaha NS10, and adapted it to balanced armature drivers, in addition to making up for some of its shortcomings. The result is a Capitol Studios-approved sound signature that is very, very close to dead neutral. Adherents of Etymotic's ER4 will tell you that the UERM has more bass than is dead neutral, and not enough presence in the 1-4 kHz region, as well as an elevated 10 kHz region, and they have their point, but the UERM is on a practical level just as good an alternative. After all, the UERM was designed to fit into the Capitol Studios ecosystem, and they likely accounted for their own studio environment. UE's second-most neutral earpiece is the entry-level UE4, which is a little more V-shaped than the UERM, but is very nice for its entry-level positioning.
For both performance and mixing purposes, I'd actually recommend a third candidate -- the JH13PRO FreqPhase. Personally, I'm not into Jerry Harvey's super aggressive-sounding earpieces, but the JH13 is definitely a rock star's earpiece. It'll give you the bass resolution to perform well with drums, but will still be neutral enough to mix (but it's not as neutral as the UERM). Its stereo imaging is top-notch, so checking pans with the JH13 will be easy.
With respect to durability, you can't really go wrong with any CIEM, to be honest. They're all designed to withstand the rigors of stage and studio use. There's a possibility that the soft material of the Westone ES tips will break down earlier, but it isn't likely. The more likely scenario is that the soft material starts yellowing and looks uglier (but it's in your ears, so the audience won't be able to see).
FYI, there is a world out there beyond the big three of UE, Westone, and JH Audio. Many are more cost-effective than the big three, and they offer sounds that are at least comparable or will even surpass models in the same price range, but if it's after-sales service you're looking for, then UE and Westone are the safest bets. I suggest you browse head-fi a bit longer to see what other options are out there. Most reviews are quite thorough with respect to how neutral an earpiece sounds, or whether it offers good bass resolution.