all labs SHOULD give you as the consumer the same results, but sadly, they DON'T.
Generally, *pro* labs are supposed to be held to a higher standard, but in all reality, a minilab with a high throughput might have better consistency than a lab that might run less film per day.
you really want to look for a lab that has these attributes:
1. high throughput(lots of customers). This shouldn't be because they're the only game in town, send your film to a good lab, not just the convenient one for you. If you can find a local lab that is consistent, all the better.
2. A lab that actually has PHOTOGRAPHERS behind the counter, not just techs(aka "lab monkeys"). I've worked in 2 labs in my life, and I have some horror stories, believe me

. Preferably people who shoot FILM, because that's where they most likely process their film. They most likely don't want scratches or other marks, and I'd venture to guess you don't either.
3. A lab that is on top of their chemistry, and is willing to show you their charts. Now this is kinda extreme, but if a lab really means business, they'll be willing to show you their charts as to chemistry changes, fluctuations, etc... I've only had to do this once, when overseas, the lab manager looked at me kinda weird, but I told him "I'm a pro"(aspiring

), and he changed his opinion, and handed over the clipboard very quickly.
I don't know really what to look for, but this is a "dumbies guide" to see if a lab is cool or not, and not hiding something.
personally, I send my film to a lab that has a quick turnaround, but I have to mail it to them.
You didn't mention as to where you're located in this great earth, but I'd assume some metropolis with a somewhat big population. Let us know where you are, there's probably someone near you that can recommend somewhere good(if there is one).
E-6 is a little more finicky, and since there are more steps(vs. c-41's 6 steps, 10 steps for commercial E-6 lines), and controls are tighter. Negs you can adjust filtration for printing, chromes(slides) are meant to really be more of an end product in themselves, so keeping an E-6 line in check is generally a full-time job in a lab. That's why so many labs have shut down over the past decade, mostly because those people who still shoot film are mostly shooting C-41, or d!g1+@l for their work(if working pro's).
the same thing with printing, the more throughput, the greater the expectation for top-notch results. Costco has long been lambasted as a bad place to go. Some Costco's are great, and I've NEVER had a problem with their processing/printing, others not so good. And their prices are better than those at a "pro-lab" too

. Just wish they could process 120/220.
-Dan