What are my options about my rear shock? I have a sort of rare Totem Shockman 29er bike, which as far as I know is assembled in my country by another brand as a lower entry point bikes. Through the two years I've had it I replaced some pieces for better ones, and now I was thinking about the rear shock; it has a stock KS Exa Form 290 coil shock (http://www.kindshock.com.cn/en/product.asp?id=164&amp;show_list=1) which measures 150mm eye to eye and has 30mm of travel, which is not terrible but it's not entirely good either and had to send to my local store yesterday because of very loud creaks on it. Having only 150mm eye to eye, I have a lot of questions:


Is there any coil shock better than this one that would fit?
Should I pick an air shock instead? Would that be worth it and a noticeable difference?
Would a RockShox Monarch RL (152x31mm) fit properly despite the few mm difference? 
Is it possible to fit something slightly larger, let's say 165mm shocks or would that be bad for the frame or the bike in general? 


Sorry for all the questions, I was a complete rookie when I bought this bike and I'm still learning a lot every day, and thanks for your help.
I can attempt answering some of your questions, without recommending any products.


  Should I pick an air shock instead? Would that be worth it and a noticeable difference?


That depends on what "feel" you want to have from your shock. If you never tried both types of suspension medium (coil vs air), you cannot choose for yourself in advance which one you you would prefer, as it is a taste question. Maybe you will feel significant difference, but it would be caused by the fact that the new shock is new and of higher tier quality. Maybe you won't notice any difference at all.


  Would a RockShox Monarch RL (152x31mm) fit properly despite the few mm difference?


Any change in shock total length will affect bike's geometry and its handling properties. Going a couple mm may be unnoticeable; or maybe it will. Also make sure that you know which mounting hardware (new bushings or bearings) you need to mount it on the frame.


  Is it possible to fit something slightly larger, let's say 165mm shocks or would that be bad for the frame or the bike in general?


The difference between 165mm and 150mm is 10%; it wouldn't come without a notice. I would not recommend doing that unless you have a chance of trying it and then going back if you don't like it.