Have u tried sitting or ride the bike you want to buy? If not, I suggest you to do so. Whether if its a 125, 250, 2T, 4T - it all depends on your budget and personal preferences such like its aesthetics.

Personally, I enjoy riding bikes. Have with me a KSR110, 2013 ZX6R 636 & 2011 Z1000. All bikes gives me different riding experience.

Breakdowns - all mechanical parts tends to wear and tear. If you are like me who does not know anything about mechanical parts - get a trusty mechanic to sort things out.

Maintenance - very relative. But a sub 250cc bikes, it is almost half the cost of what you are paying for a car. For bigger cc bikes, it is 1:1

Top speed - for a zx150. No idea, but i suppose a 150km/h from a stock bike?

Size - Between the 150 & 250, 250 looks bigger due to the fairings & engine.

Last but not least, quite a number of zx150 & 250 running around. Should not be hard for you to test ride one. My Z1000 is up for sale if you want...