I have the same doubt as you, mostly between the the SC-550 and the SC-custom. As you probably already know, the first is more similar to a Gibson Les Paul and the second to an ESP Eclipse.
After a couple researches, I've learned that the difference between the two are:
- Custom has Grover tuners, which are supposedly better than the unbranded tuners on the 550
- 550 has a bone nut, Custom has a graphite nut: which one is better is debated, as long as they're cut properly it shouldn't matter much, however if you prefer one over the other swapping a nut is quite cheap and easy, so it shouldn't be so influent in your guitar choice
- Custom has 2 volume + 1 tone, 550 has 2 volume + 2 tones
- Custom pickups are splittable, 550 are not
- Custom has a 6 mm thinner body and also a thinner neck: it's up to preferences, but usually a thinner neck is more suitable for fast runs and scales, while a thicker neck feels better for chords, my opinion is that it doesn't take much to adapt to either one
- 550 has better high frets reach thanks to a slimmer neck-body joint
- nut is 42 mm wide on the Custom, 43 on the 550 (shouldn't make a big difference, if any at all)
- not sure of this because I found it referring to old Wilkinson pickups, I don't know if with Roswell is the same...but still: the Custom bridge pickup should be way higher-output than the 550 (13.5k against 8k ohms), this means it's more suitable for metal / hard rock or high gain sounds. A lower-output pickup instead is more responsive to volume adjustments and cleans up better when you back off the volume a bit.
Other than that, there are some minor or aesthetic differences, which are up to personal taste:
- the curved archtop on the 550 is more pronounced
- the 550 should have a higher quality maple top
- the binding on the 550 doesn't surround the headstock, as it does on the Custom
- vintage tuners on 550, modern on Custom
- strap locks on the Custom, regular on the 550
After all of this, I have basically decided for the Custom, hoping it's available soon, mostly for the electronics. I like that it has split coils and I also prefer the 2+1 knobs, because I find the 2+2 less immediate to interact with, but that's of course just personal preference. Also, the hotter bridge pickup (if so it is) suits better my hard-rock / metal style. Last but not least the Grover tuners are a nice plus.
I prefer the 550 only when it comes to aesthetic - yeah, the Paradise flame looks great - but that's too little for winning.
I can't help much with the 450 because I have never considered it in my choice, since it just has worse hardware compared to the 550 (worse tuners, worse switch, plastic nut, unshielded electronics and so on), except for the coil split, which is present on the 450. However If you're not on a budget, I think that the 550 is just worth the extra money over the 450, so I'd simplify your choice between the 550 and the Custom.
One day I'm all for the 550, and the other day I'm all for the Custom.