The Custom 3 is a nice earphone. It is a discontinued product though, but it's still available on eBay for around $150 which is a decent price for their performance. My only real personal gripe for the Custom 3 is that Klipsch, for whatever reason, chose a very bad cord. It's very thin, tangles easier then fishing line, and it just a stupid mess. The drivers used in the Custom 3 are quite good and match very well. Klipsch must made some dumb design choices that eventually led to the discontinuation of their best sounding IEM product. I'll toss out a reshell of the Custom 3 is a viable option for a better fit and better cabling. The drivers and x-over are good enough to be worth doing so.
For no hassle, the Mosters would be a better choice. I haven't listened to any of the Moster Turbines, so I can't comment on their sound. The Gold is supposed to be pretty darn good though, so I would have a tough time seeing someone not like the product.
Now the Custom 3 and MTPG aren't your only earphone options. At a similar price point things like the RE252, CK10, Triple.Fi 10, or SA6 are all available between $150-$200 new or perhaps a little less used. Frankly, there's a lot of options out there.
The correct earphone choice is the one that has the sound characteristics you want to hear. In order for us to point you to something very fitting to your tastes, you'll have to describe, in detail, the sound characteristics that you find important and the overall sound signature you're trying to step towards. Some examples of characteristics are:
tonality: warm, natural, bright
level of detail: smooth to sharp
impact/energy/dynamics: laid back to punchy/energetic/explosive
sound stage depth: intimate to expansive
locational cues: general sense of left/center/right to exact point in mental space
thickness of note: thick and full bodied to airy/delicate/short
These are just some characteristics, but every single earphone out there will sound a little bit different. The whole goal for you is to find the one that best fits your personal taste. That can be quite a challenge unfortunately, and there are a sea of earphones to wade through. Luckly, there have been a lot of mass earphone reviews by a number of forum members. I would suggest you at least read through those first to get a feel for what's out there, how they roughly stack up, and figure out how they each sound in a rough sense and if any are described like your personal preference.