Hi Peter,
I can't help you with congas, but I can on djembes. The answer is:
it depends on what kind of voice you want to get from the djembe. Do you want to play lead? 1st or 2nd djembe? Are you using it to accompany soft acoustic instruments or are you looking for a loud sharp cracking djembe to play with traditional West African Sangba orchestra and dance?
I play lead djembe for a West African ballet company ad I own several djembes which have heads between 12" and 13"in diameter and thebetween 23" to 26" tall.
THERE ARE NO COMMERCIAL BRANDS OF DJEMBES WORTH BUYING!! If you want a good djembe, it's a hand-made item. I personally prefer the djembes from Ivory Coast or from Guinea. I deal directly with the West Africans, so my price is pretty good. If you are looking for a good servicable djembe, there are a number of people selling them on-line, most of it's junk, but good quality CAN be had. Oe place that I know of that regularly sells very good quality Ivory Coast djembes is African Rhythm Traders.
www.africanrhythmtraders.com
They may not be the least expensive, but I've always found them to be quite honorable and have high quality products.
Expect to spend between $250-$450 for a djembe that is of "instrument quality". You can get a djembe that was hacked out in Ghana for $50 at TJ MAXX or Marshalls and you can use it as a decorative end table, but you wouldn't want to play it.
I also own a Remo synthetic djembe (12" head) that I have tuned so that it almost sounds like a big doumbek that's trying to be a djembe and I only play it at venue that are outdoors when it's damp. It's tolerable, but it doesn't sound that great. Toca,LP and CP djembes are just as awful (although they make really good congas and bongos).