I understand why people go with other vendors and I don't think there's one right answer. For me personally though I go with SS for the most part.
Cons:
Tax- I'm in California (though tax may be true for everyone soon)
Nrd- I don't typically cancel if I put in an order so not a big deal for me
Higher shipping
Pros:
Get it early- Lately this can mean much earlier. I've had HT Spidey for months while some haven't even got there's from BBTS yet.
Easy access to replacement- in the event that I need to get a replacement they've always been helpful. I know this isn't everyone's experience though.
Payment plans- I usually split between these and full pays depending on what my payment calendar looks like.
RP- I've gotten a few figures off RP alone.
Quick shipping- high priced, but because I'm in CA once something processes at SS I usually have it in 2 days.
Like I said that might not be everyone's experience, but those are the reasons I've stuck with sideshow so far.
You'll save more on tax and shipping than RP when you buy BBTS. You have to pay extra to have outer packing with SS; your box always arrives fresh from BBTS unlike SS where your box looks like it's been used for soccer practice. I don't care about getting it early as long as I know I have not been swindled on shipping and tax and my box arrives looking like it just got sealed straight from HT. Unless you want to keep paying more to SS for inferior service, BBTS is the only right answer if you're in California.