Haha, I've got some experience doing this, that's all. There are plenty of guys on this forum that absolutely amaze me. They've got specific expertise and are happy to share it, too. That's what this is all about.
To break it down for you, though, injector selection factors in several variables like:
a) Cylinder head design: Picking the correct spray pattern is critical so as to put the fuel at the backs of the valves rather than to produce a splattering/dripping mess.
b) Engine fuel efficiency & detonation characteristics: A sufficient flow rate will support safe power with an appropriate air/fuel ratio.
d) Injector response time: Some injectors are slow to respond while others are exteremely fast. Getting the right response time is important so that the ECU's open/shut "instructions" can be followed precisely.
e) Fuel pressure: The various technology types are more and less tolerant of high pressures. Some can handle no more than 5bar at the rail (such as you'd get with a 3bar regulator and 2bar boost, for example). Others can take 150bar or more!
f) Engine management system type: Injectors are typically either high or low-impedance. A factory ECU will always take high-impedance. Install lows and the injector drivers will fry immediately.
g) Atomizer/metering valve type: Injectors may use any of several different types of atomizer and metering valves: pintle (older Bosch), vibrating disk (Lucas), ball & seat (Rochester), and annular orifice (Bosch & Genesis).
The vibrating disk-type injectors are probably the very fastest in terms of response time. They're also extremely sensitive to (and often lock open/shut at) elevated fuel pressures. Their spray patterns are okay for single intake valve engines and horrible for heads with multiple valves. The ball & seat types feature wonderful atomization, but become uncontrollable at high pressures and high duty cycles. The pintles injectors are reasonably quick, can handle a fair amount of pressure, and can have either fine atomization or fire very tight streams (depending on the design engineer's requirements). All these injectors are generally non-directional, though, meaning that only a single cone that fires straight ahead is emitted. The hot ticket are the annular orifice injectors that are very fast, take big pressure in stride, and can spray in different directions at once.
Ya, I could rattle on and on about this stuff.