A LDC mic will generally give you a fuller more "open" and "airy" vocal sound when recording, which is usually desired.
The Studio Projects stuff is definitely good for the $$$. They just revamped (possibly improved?) a lot of their stuff too, which was already considered very good (again, for the money).
Another brand worth looking at is Cascade (
http://www.cascademicrophones.com/). Their Fat Head ribbon mic seems to be garnering some fanfare... less than $150. I was using a pair of their M37 SDC's for drum overheads at my bands practice space for a bit... they sounded pretty good (I actually got them for free with another purchase). Only problem was that our drummer wails too hard sometimes and they don't have a -10dB pad built in. Ended up getting a pair of Studio Projects C4's to replace them... and they are an overall improvement (also have the pad and LF rolloff options).
I have a Joemeek JM37 LDC... not very impressed by it at all in the couple times I have used it.
Also have a couple MXL's (770, 990). Again nothing impressive. I've seen people say the Mogami edition they have is decent, but I have no experience with it.
The Shure KSM (preferably the 32, the cheaper 27 seems decent as well) series is worth looking at for an LDC. The SM7 is a classic (if you can spend a bit more).
For the AT's, the 40xx series get a lot of decent reviews... not so sure about those 20xx you linked to.
I'm pretty jaded these days though, as I recently picked up a Red Microphones Type A with a couple capsules. No other mic I have owned/touched comes even remotely close. Then again, it probably costs nearly the same as all my other mics combined (and I have 15 others).
As with everything, you get what you pay for. I can tell you that a better mic will generally require less work to get good sounds (if you know/learn proper mic placement techniques) and also make things easier to mix. Try before you buy if you can.
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