good question! a 520nm laser is generally a direct diode laser, meaning it has one electronic component generating the laser light, so it doesn't have warm up time and has very little power fluctuation compared to DPSS, imagine it like an LED that emits coherent polarized light, unlike a normal LED. they do have drawbacks though, but nothing terrible in terms of the green diode lasers.
DPSS lasers (the 532nm green lasers) or "Diode Pumped Solid State" lasers are more complicated. They work by using laser diodes to excite a laser crystal in order to make it emit laser light. this is much less efficient, as energy is lost while "pumping" the laser crystal (pumping is the term for what i just described). DPSS lasers are less efficient, sensitive to temperature, and less stable than diode lasers, however have excellent divergence and have very narrow linewidths, making them very useful for lab applications, but less suitable in pointers and handhelds than diode lasers. 532nm lasers in particular work like this:
a laser diode emiting at 808nm pumps a Nd:YVO or Nd:YAG laser crystal, which starts lasing at 1064nm, and then that light is passed through a KTP crystal to double the frequency to 532nm, a green color. during these steps, energy is lost, making them much less efficient.
overall, 520nm will be much more stable and efficient, however wont have as nice of a beam, and is more expensive. hope this answers your question well. be safe too, get some safety glasses, i recommend these for a relatively low power first time laser:
http://www.amazon.com/HDE-Protectio...38134756&sr=8-1&keywords=laser+safety+glasses but in the future if you get more powerful lasers, never cut back on safety.
i recommend a simple google search on these in case you were wondering:
Nd:YVO stands for Neodymium doped Yttrium Orthovanadate
Nd:YAG stands for Neodymium doped Yttrium Aluminum Garnet.
KTP stands for Potassium Titanyl Phosphate.
