Parts Spending. Do it wisely (Rear derailer)

I: Why do you change from 7 speeds to Deore 9 speeds groupset.
The person I met in LBS: Because it is 7 speeds!?

 
Impulsive spending
Occasionally I walked into LBS and walked out with something I didn't have intention to buy at the first place. It could be something that I never realize I need it until I saw it. It is like window shopping in ACE Hardware.  However my impulsive spending is limited to cable end, cable housing end, chain lube, shifter cable, brake cable and other consumables.


Parts upgrades
When the part deteriorated in performance or functionality, I'll go for better part replacement. Say that my RD need replacement, well at this moment I have 1 weak reason for it which is the jockey wheel becomes pointy. Since current RD is Shimano XT, better part would be XTR but exorbitant price may not allow my hands on it. So very likely I'll go for XT of the newer model.

For a start, there are 6 variants from current Shimano XT RD which make my choice become harder.  Check out the following: -

1. Top normal, Shadow, Long cage, Direct routing (RD-M772-SGS)

source: bike.shimano.com

2. Top normal, Shadow, Medium cage, Direct routing (RD-M772-GS)

source: bike.shimano.com

3. Top normal, Long cage, Straight pull (RD-M771-SGS)

source: bike.shimano.com

4. Top normal, Medium cage, Straight pull (RD-M771-GS)

source: bike.shimano.com

5. Low normal, Long cage, Straight pull (RD-M770-SGS)

source: bike.shimano.com

6. Low normal, Medium cage, Straight pull (RD-M770-GS)

source: bike.shimano.com

Top Normal - resting pulley is on top i.e. smallest cog (eg. 11T). Downshifting on the shifter requires pushing your thumb on the lever.

Low Normal - resting pulley is on low i.e. biggest cog (eg. 34T). Downshifting on the shifter is by 'release' action by your index finger

Shadow - Non-protruding RD.

Direct Routing - cable comes from top to down.  I believe this overcoming issue with Straight pull.

Straight Pull - cable comes from behind to front, thus the loop with potential to hook on something on trail.

Medium Cage - for chain capacity up to 33T. Eg. 32T/22T chainring, 11T/34T cassette = 33T

Long Cage - for chain capacity up to 45T. Eg. 44T/32T/22T chainring, 11T/34T cassette = 45T

Chain Capacity (aka Total capacity) - result of (biggest cog - smallest cog) + (biggest chainring - smallest chainring)


Decision making
Long or Medium Cage
I'm currently running 36T/22T chainring and 11T/32T cassette on my Kona Cindercone thus my chain capacity is 35T.  This means I can only use Long Cage if I were to use 36T chainring and 32T at any point of time. 
Shortlisted: RD-M772-SGS, RD-M771-SGS, RD-M770-SGS

Top or Low Normal
Since I'm used to Top Normal, using Low Normal requires some adjustment for my brain to register and react subconsciously.  As a matter of fact, all my bikes run Top Normal thus Top Normal would be my choice.
Shortlisted: RD-M772-SGS, RD-M771-SGS

Shadow
Shadow is associated with Direct Routing.  While I like the RD to be less protruding, it comes with a perk of Direct Routing, features that I believe address issue with Straight Pull.
Shortlisted: RD-M772-SGS


Verdict
Top normal, Shadow, Long cage, Direct routing (RD-M772-SGS)

source: bike.shimano.com