As far as light output there is essentially no difference. The big difference is the current that each of the 2 types draw.
5V strings or strips have individual leds running off 5V. Therefore every led draws 20mA with about 3V (very roughly) across each led. There is 2V that is dropped across the series resistor and that dissipates heat.
12V strings/strips typically have 3 leds of the same colour in series. Each of the leds has the same current flowing through it and the extra 3V not dropped across the leds is dropped across the series resistor.
If you compare 150 5v and 12V leds the differences are.
150 5V leds. total current equals 3 Amps. Power equals 15 Watts
150 12V leds. total current equals 1 Amp. Power equals 12 Watts
There's very little power difference but it's the current that is an issue when using RGB leds. The current is 3 times as high for 5V strings so the voltage drop is 3 times higher when using the same sized wire. There's still no problem until you find that the 3 different coloured leds have different voltage drops across them. What this means is that 1 end of the string will have a different coloured "white" when all the leds are on. This usually means that the end closest to the power is lovely and white whereas the far end is pink. 12V and 24V strings are far less susceptible to this effect than 5V ones.