I'm very much a fan of the "piano chime" of Stainless Steel strings as well, so take that with a grain of salt. My favorite strings for my fretted basses are D'addario ProSteels - wonderful "chime" but not at the expense of other areas. IMO they've got strong low-mids and low-end but not enough to overwhelm and be "boomy". Same with the high-end, present but certainly not enough to "take over". I'm also a fan of the "feel" of the strings - they're not "finger-rippers", but jut enough "grip" to keep your fingers from sliding off, and the perfect amount of string tension to keep my low B & E nice and tight without having to use heavy-gauge sets.
I also like Spector's own Steels, but honestly need to spend more time playing them before I can give an honest assessment about them - they're going on at the next string-change on my fretted 5-string.
Obviously there's the USA vs Czech Republic Quality argument, but from a purely practical POV, the ReBop DLXs (Euro Bolt-on) have an Alder back and Figured Maple (FM) or Zebrawood (on the "EX" Exotic Top models) front compared to the USA's Mahogany back and [Invalid or Expired Link Removed] depending on what Stuart's made up - typically the "standard" is Figured Maple for the "FM" models and Spalted Maple on the "EX". Body-wise I've found the US bolt-ons to not have quite the curvature/concave bodies that the NS design is known for - they're a little "flat" for lack of a better term, whereas the ReBop's have the standard fully curved NS body shape. The ReBops have the Classic Spector Locking bridge with aluminum base plate and Gotoh tuners in black, while the USA has a custom Hupshot bridge based off their A-style also with an aluminum base plate and brass saddles, along with Hipshot Ultralight tuners. Both have the 24-fret Spector neck/fretboard with Crown inlays in Mother-Of-Pearl, Rosewood on the ReBop, Pau Ferro on the USA. To my ears the ReBops are a mix of the classic Spector tone combined with a good-quality Jazz Bass, due to the bolt-on nature of the construction and body woods, where the Mahogany in the USA version gives it a more "midrange" tone - I hear Mahogany as a very "middy" tone wood that needs a bright top to "even out" the sound. Still, I'd like to pick up a USA bolt-on one day to compare the differences...
I'd say the biggest difference is the electronics packages currently. The ReBops come equipped with EMG-HZ Passive Pickups and the Spector OEM TonePump 2 boost-only preamp, while the USA bolt-ons come standard with EMG-CSTW pickups (Dual/Single-Coil switchable) along with an Aguilar OBP-3 3-band active preamp. That's a pretty big jump in electronics, and as much as I love my ReBops (EX Zebrawood-topped 4-string, Flame Maple 5-string fretted and lined fretless) I'm currently having 3 sets of Nordstrand Big Single pickups custom-wound to be installed with Aguilar OBP-3 preamps, which added a significant amount to what I've spent (even though I purchased mine about 2 years ago before a major price increase on Spector's part).
...and now I've just re-read your post wanting a comparison between the USA bolt-on and the Euro... I ASSumed the "Euro bolt-on".Someone can take over from here, but this is a pretty good comparison between the two bolt-on models. Dammit...
Short Version: differences in body woods and construction. I think the biggest issue is whether you're a fan of bolt-on or neck-through construction. I myself am not, so if/when I do decide to upgrade to USA Spectors, it will be to the USA bolt-on models.
You may have missed the neck through part but you explained everything really well. I wasn't aware that the USA bolt on wouldn't have that curve, I like that, I may have to send it back for a euro model..
Someone can take over from here, but this is a pretty good comparison between the two bolt-on models. Dammit...
Short Version: differences in body woods and construction. I think the biggest issue is whether you're a fan of bolt-on or neck-through construction. I myself am not, so if/when I do decide to upgrade to USA Spectors, it will be to the USA bolt-on models.

) DA nailed the differences between the pickups and electronics, and the $400 difference is going to a better grade of Figured Maple (which on Spectors aren't so much "the top" as they are "half the freakin' body - I was amazed when I got my Zebrawood-topped ReBop DLX EX and saw that it took up as much room as the Alder "back" did, and is sonically noticeable) as well as a major financial as well as tonal upgrade in the pickups and preamp, from EMG-HZs and a Spector TonePump Jr. to EMG-35TWs and Aguilar OBP-3. Toss in the "USA Spector" aspect, and should you have to sell it you're likely to get a much better return on your investment... and you're talking to someone who *loves* his euro-made ReBops, but won't ignore facts regarding the resale value.