In the last five years I have probably bought dozens parts or kits or upgrades or something from each these two companies. I probably spent clos to $12K with Lingenfelter and maybe a third that much with MTI. I respect them both but they are very different.

First, what they have in common: they are run by people who really like fast cars (particularly 'vettes) and truly want to help you: when you phone they aren't just taking an order.
Second, both places know quite a lot about 'vettes and hot rodding and their advice is pretty good (they will tell you if their product isn't best for you). Third, sometimes the guy you want, or the one the person on the phone says you should talk to, can be tied up for most of a day. And you have to call them back - they're busy and sometimes forget.

But they differ:
MTI is clearly a company with an edgier focus: I think what they love most is max power on a budget, even if it gives the car quite an attitude and makes it something of a beast to drive.
They have good parts and good advice.
Their response and shipping is absolutely terrible. Twice, they mailed me my order twice: I ordered one set of Kocks headers, but got two identical shipments. Another time it was two catch cans.
(Each time I shipped the extra back - probably could have kept it and they never would have noticed, but . . . ). Other times, an order for, say six small things arrives but one is missing, etc.).

Lingenfelter is a bit different, and more my kind of people. They tend to value good street manners more than MTI, in my opinion, the type of people who see no reason why a car with twice the horsepower of stock should not idle and drive at low speeds as smoothly as stock. My impression is that they prefer to spend money to do things in a top-quality manner rather than cut any corner. On the other hand they would always tell me where I could cut a bit of money out if I wanted, and what the consequences might be (I never did).

In particular, they could answer difficult questions about shaft mounted rocker sets and heavy duty pushrods, etc., that no one else really seemed to know as well.

their shipping is first rate and in every case I got what I ordered and it was first rate stuff: Lingenfelter never sold me anything that was not absolutely top drawer or did not arrive as ordered. . On two occasions when I returned stuff, they refunded correctly and promptly (not so summit Racing - never got my money back).
They do have only one flaw: they are often optimistic about when a new part or kit would be available: I'd order it and they'd sway about two weeks and it would be six. I think this is par for the course, though