For battling, I've always been a fan of the plastic tracks. 

The lighter weight is easier on the gearboxes and the battery tends to have a longer life.   If you're able to keep the weight down on the tank, then you can float across the grass and not get bogged down.

The downside of plastic track is that will stretch a little on the hot days and can occasionally cause the track to slip off.

the only tamiya track i have a problem with is the KT because its a 2 link type with no meat for the pins sit in
Tamiya Tiger 1 are 1 link and thick where the pins sit
Tamiya Sherman come already together with the outer parts that the drive sprocket grabs seems to be metal and the pins are not easy to remove from.

I guess it comes down to what plastic the companies use for the track and the cheaper tank companies will use cheaper plastics that will stretch more from the heat then just from use and time

Having only ever driven around a metal lower/metal track heavy Taigen KV-1, I'm biased regarding my track opinions.
I believe I understand the differences however and please correct me if I'm wrong.

Plastic Tracks
Pros:
- Cheap, 1/6th the cost of most comparable metal track versions...
- Light in mass. Drivetrain requires far less effort/torque to move the tracks, especially from stationary point.
- Light in mass. May not get traction due to lack of ground pressure/engagement with surface.
Cons:
- Far easier to stretch and break on outdoor terrain due to side loads/debris.
- Visually less accurate (track sag)

Metal Tracks
Pros:
- Heavy in mass. Usually better traction on surfaces due to mass/friction. Heavier vehicle mass = ability to go through longer grass easier.
- Usually very robust. Far less susceptible to side loads/debris.
- Visually better (Track sag)

Cons:
- Cost
- Heavy. Requires gearbox upgrades from plastic due to mass, plus the mass adds more stress and strain on the suspension. Higher flex possibilities in tank chassis. Aluminum tracks are roughly 3x the mass of plastic.

Metal in longer grass does not run better at all believe me. Michael and I sometimes look at each other and say why do we spend all this money on metal upgrades on motors, tracks, gearboxes, when Scout runs circles around us battling with her little Pz. IV? LOL!

Yes, metal makes the tank move prettier but can cost you in the battle....

My T-34 loves the long grass, it built so light it sprints across the top of it.  The guys that went Oshawa can vouch for it, it was running every where.

my Pz IV has the metal winter track and it can run just as fast as any but i guess being wider helps with the weight. If anything the clark battle damage holds it back

Metal tracks on rear wheel drive tanks , since they only run on 3rd of the track anyways , plastic on front wheel drive , especially german tanks with super wide tracks

Plastic for me, especially on grass surfaces.

Metal for me.  I don't so much battle as much as do marathon runs with my tank on parks, paths and sidewalks.   This means that the track has to be sturdy enough to withstand moving through a number of different surfaces some of which like hard gravel paths and rough concrete can be pretty rough on plastic tracks.