At first I thought I was going to get full size servos for this project, but the lowest price I saw for those was around $10 per servo, which would have added $180 to the project before it even got started.  I did not want to make that kind of commitment never having worked with these components before.  I did a lot of research and have tried to summarize it below.

Here is a look at the current outcome of having chosen metal geared MG90s micro servos:

Considerations

Cost/Quality – Servos are another type of purchase where you get what you pay for.  I feel like the choice was sort of “expensive” vs. “disposable”.  There are also robotics grade servos, but those were definitely out of the question.

Gear Type – I wanted the more expensive metal geared servos instead of plastic so that the gears don’t strip/slip under the weight of the robot. I’ve heard these can get hot but I can’t test that yet.

Brands and Models – I’ve heard Hitec and Futaba are the better brands.  Many people have strong negative and positive opinions about brands and models of servos.  This is in part due to the fact that the market contains a lot of knock-off products and a lot of servos with really bad quality control.  Many arrive faulty or fail quickly.  On top of that, I found most of the discussion about servos seems to center around keeping expensive RC planes and helicopters in the air.  As I understand it, there are also a lot of issues with online hobby shops holding people’s money hostage while they obtain their stock from overseas.  Lastly, using servos to carry the weight of a robot is an inherent abuse of their design.

Physical Size – Standard size servos with metal gears cost too much to be “disposable”, and I didn’t want to spend enough to get a quality brand/model at that size.  I chose the micro size instead.  The micro servos are lighter and use less power too.

Voltage and Torque – For testing I was able to power one servo using the Arduino, but it definitely can’t power 18 of them.  I don’t know how many it can power safely, but I think the answer is very few.  The more voltage you supply to a servo (within its specified range), the greater its torque (strength).  Torque is one of the more important factors for walking robots since they need immediate and constantly available support for their weight.  Running servos at their maximum voltage will wear them out quicker though, so I chose to run mine at 5V which is in between their minimum and maximum values.

Accuracy – I’m actually minimally concerned with this, as I want my robot to have a little personality anyway.  I plan to accommodate for any detrimental imprecision with code wherever possible.

Practicality – I wondered was it even practical to use micro servos?  They’re not famous for doing heavy lifting.  It turns out yes!  There aren’t as many examples of these in use, but some of my favorite robots are using them.  Sometimes they are a little shaky in certain positions, but I will try to design and code around that as well.

Examples

Pretty much my favorite robot:
https://www.youtube.com/watch?v=-KDnMeRvkqo

Amazingly fast and very smooth:
https://www.youtube.com/watch?v=HlxTxyLp2qk

I think this one is a kit:
https://www.youtube.com/watch?v=MjTJpvNxVcM

Nylon geared:
https://www.youtube.com/watch?v=yFHzD1PYoY8

Decision

Given the quality issues, I assume I will be replacing servos periodically, so being able to return them and get new ones quickly and cheaply is my main concern.  With hobby shops and eBay ruled out, I ignored a lot of negative comments about the TowerPro MG90s and ordered it anyway because it was the only servo that met the requirements AND could be ordered via Amazon Prime for $5 per servo.

TowerPro MG90s Micro Servo Specs:

Dimensions: 22.8 x 12.2 x 28.5mm

Operating Speed (4.8V no load): 0.1sec / 60 degrees
Operating Speed (6V no load): 0.08sec / 60 degrees

Stall Torque (4.8V): 1.8 kg / cm
(also seen listed as 2.2 kg / cm)

Stall Torque (6V):   2.2 kg / cm
(also seen listed as 2.5 kg / cm)

Temperature Range: -30 to +60 Degree C

Dead Band Width: 5usec
(also seen listed as 2usec)

Operating Voltage: 4.8 – 6.0 Volts

Weight: 13.4g
(also seen listed as 13g)

Links

Types of Servos and How They Work
http://www.societyofrobots.com/actuators_servos.shtml
http://www.rchelicopterfun.com/rc-servos.html

Tips for reducing servo failure:
http://forum.arcbotics.com/viewtopic.php?t=193