Shimano XT from Rose bikes - job done
What he said. Even Deore is better than the crap Avid sell
Rose bikes and bike-discount.de are awesome places. Prices aren't ALWAYS cheaper than the UK for eveything, but they tend to be cheaper for a lot of components. The prices are often a lot cheaper in Euros than they are in GBP before you even convert.
I just picked up some Shimano SLX brakes from Rose bikes. I used 2 x €10 voucher codes for further discounts. Even with shipping I saved about £50 on UK prices on my whole order. You can get voucher codes by liking them on facebook and subscribing to their news letter. You can also get a loyalty/points card for free.
I have owned Avid Juicy Brakes as have some mates of mine. My advice would be that if they are working, leave them alone. If they are working ok ish, leave them alone. If they start to annoy you or the performance becomes unacceptable, then swap them out. My friends and I have had problems once we start tinkering with Avids. They do seem to be more high maintenance to actually setup and keep working well. Problems with rotors rubbing is common. The bleed procedure is not the most simple, but easy when you know how. However, in my experience, I completely serviced my Juicy 5's with new piston foot and seal kit, bled them all from scratch and I still had problems.
Everyone seems to rave about Shimano current generation (and last) brakes in that they have great bang per buck and "just work". They don't weigh the earth, and not over engineered and seem more simple. They also use non corrosive mineral oil which is nice.
Lowest level is Deore, then SLX, then XT, then XTR highest. I'd recommend SLX. The weight difference between it and XT is reported to be minimal at best. There are even pictures on the net of people that have weighed them and they are identical weights. The XT you gain a pad adjust knob and instead of a split pin to retain the pads like on SLX, it's a threaded bolt. The XT also has chromed reservoir covers on the levers. Some like this. Others feel they look cheap and tacky as the finish looks a bit "chrome spray in a rattle can".

SLX with the non finned pads can be had for around £113 a set (without discs). I personally wouldn't bother with finned pads once you see the cost of genuine replacements. You don't need them to brake well. Yes they do "work" though and get hot, although any pad (and hence heatsink attached to it) will get hot I don't think this proves much other than it takes away some of the heat disapation from the disc to the heatsink. Other manufacturers get by without heatsinks so I'm not convinced and think it's a gimmick, but then I am a grumpy old man.
Deore are still excellent though. Any of the above would be an upgrade over Avids,
if your Advids are playing up. When Avids work they are fine.
Hope this helps.