Core-to-core is only way I splice, mostly b/c its easier.
I've only had one dealing with rig-rite. I needed replacement sheeves for my deck organizers. However, the old ones were too messed up to meassure accurately. I tried to explain this to them and they didn't seem to care. Basically if I didn't have the sheeve diameter, I was SOL. The woman I spoke with was very rude, and short with me over the phone. Didn't seem interested in the type of organizers I had or the fact that I had the bushings that were in the sheeve to indicate the width. Or what type or size boat. No sheeve dia. = no sheeve. I hung up with the horrible feeling that I'd be dropping a few hundred on new organizers.
Then I placed a call to Garhuaer (about 5min later). That asked me what boat I had and the bushing measurements, then sent me what they thought would work. They spoke to me over the phone like I was buying thousands worth of new gear, and made me feel like a valued customer (damn near royalty). Something like $15 later, I had new sheeves that work great.
Night and day difference. Couldn't be farther apart from each other. Now, if Garhauer would make standing rigging, I'd be a happy sailor.
(to be fair, I have only one complaint with Gahauer, if you email them, the only response I've recieved is to please call "so n' so" for help selecting something. This goes for any question, like overall dimensions of a product. I don't need to speak with an engineer to measure something, just a tape measure.)
Wow, that was a longer response than I thought.