There are pros/cons with all of them. I'd never had an issue with Ecklers/Classic Chevy until recently; they sold me '55-57 spring perches for a new differential I was installing in my Nomad, and AFTER welding them on, I found out they were 2-1/4" wide rather than the stock 2"... and thus I couldn't get the U-bolts around them and thru the holes in the lower bracket (springs and the holes in the brackets are 2" wide, as are the stock perches!)... After wasting many hours installing them, finding out they weren't 'right', wasted many more hours trying to get someone at Ecklers interested enough to at least acknowledge the parts are wrong and tell me they would be correcting that (so others aren't screwed as I was). I worked thru 4 or 5 different calls to *different Customer Service Reps - all with no last name- and NONE who ever followed up), I finally called and told them I was not getting off the phone until I talked to their CEO.. I was steamed! I didn't get their CEO, but finally they transferred me to a VP in New York, who actually works for Mac'sAuto (a Ford parts supplier) whom the Ecklers owner had recently purchased.. His name is Craig Udell and HE actually listened and tried to fix the problem. He gave me a credit for the parts (a small fraction of my cost/effort on this issue, but at least he did something).. and he said he would make effort to 1) improve their parts descriptions, and 2) improve their Customer Service...
Cons for Ecklers: extremely poor non-knowledgable Customer Service (if you ever have problems)... Their order takers are friendly, but only know the limited info in the 'parts description', which is lacking.
Pros for Ecklers: Discount for 'members', and they also will help you out on shipping/price for any parts advertised on ebay as (free shipping) but ONLY IF you call them and place your order.
Ol'55 (East Coast Chevy):
Pros: The guys there are very knowledgable and will share their knowledge; they know MORE than the parts descriptions, so just ask them for their recommendation. If you don't know the details of what you are ordering, ask them for their recommendation.
Cons: Unless you pick up the parts at a show, smaller parts orders will cost you more due to the shipping... but I don't think they pad their shipping prices as some do.
For ANY vendor, I recommend ordering only Parts that you have experience with, or that has been recommended to you by others you trust. Do NOT assume that just because they say 'show quality' or 'made to fit your Trifive', that the parts actually are, or will... With repro parts, 'fit' and quality are almost always an issue...