I am going to start an arguement here but hey what the heck. This is my own personal opinion
inexpensive ( talking about non branded china made stuff ) servos are a waste of money unless you have a very short barrel tank. Shorter barrel = less stress, less weight on the servo. KV-1 type barrels are ok for these.
Any other tank I suggest you get the best metal gear servo your budget will allow, servos are cheap in general anyway with a cost of less than 30 bucks for a good one.
Why? well it takes time to place that servo inside your turret, it takes effort to replace a broken servo. Plastic gear servos can strip if there is too much pressure put on them and sometimes just the weight of the barrel after many firings is enough to do this but usually it is because you drove through some grass, turned the turret and hit a building etc. I keep driving the long King Tiger barrel of my tank into things and ruining servos. That is until I used a high quality metal gear servo, ever since no repairs.
Things happen driving around and in the heat of battle. nothing will stand up to a metal gear servo.
Spending an extra 10 bucks here can save much time and maintenance later.
Do some investigative work before you buy, every plastic gear low cost servo has a stronger more reliable metal gear servo available in its family for just a little bit extra. It is well worth the investment.