I have done a bunch of research on the topic of 12 volt vs 5 volt pixels as this will obviously affect both what pixels I buy as well as which PSU (power supply units) I will need. Having said that, I sense it is all based on what you plan to do with them and how comfortable you feel with doing power injection. I even saw someone on a forum mention it is like the age old Ford vs Chevy discussion, which seems very appropriate.
From research I have gathered that 5v and 12v pixels are pretty much the same when it comes to what power they need to run properly. If they do not have enough power they will apparently look red or pink when you try to make them full white. Being colorblind, I may not notice if the pink is really light. I will have to depend on my family to point this out. Anyways, the pixels from both sets need around 5 volts to run at full white… but the 12v pixels use buck converters/resistors to step down the power. This would mean that power is being “wasted” and I suspect some level of heat is generated as a byproduct. The positive is that power loss over a lot of pixels/wire would be less of an impact for 12v given you have a lot of overhead to lose… where 5v dropping just a volt is not good for anyone.
Quick aside on power injection: I mentioned the term before, but for those that do not know… power injection is where you not only have power coming from the original source (controller or psu) but also run power to a point further down. Think of this like where people hand water out during marathons along the way, so that you can replenish your fluids… otherwise you go down. This keeps the voltage up for your pixels no matter the length of the run or number of pixels taking away that power. Read more about power injection here. I realize my marathon analogy totally breaks down when you inject at the end of the string also as you are not getting water at the end to help you 1 mile ago. Oh well.
Now, the 12v pixels can have longer runs before they need power injection because of all that “excess voltage” they have from the start. From everything I read, 100 pixels seems a safe bet and many talk about upwards of 120 or 140 before needing power injection on 12v. Conversely, 5v pixels typically need injection around 50 pixels. This means if you want to do a megatree with 50 pixels per vertical string, 12v will get you up to the top and back down another (known as zig zag) on a single controller output. Using 5v, you would have to inject power at the top for every string (or wire from the top and inject on the ground)… plus if you do not want to inject that means 1 output per 50 pixel string. That would be a lot of outputs for a megatree.
Additionally you have to look at power consumption and waste. 12v power supplies tend to run around 30 amps where 5v psus can do around 60 amps. Looking at a calculator such as this one I can see that running a 900 pixel tree in 5v can be done on one typical 350w/60a power supply. The 12v would actually require another psu as I am over both on amps and watts. Keep in mind that different vendors have different amp/watts for their pixels, so if you use a calculator you need to look those up or test your pixels on the bench.
To me this means that I have to choose between more power injection work/wiring on 5v (unless I use more controller outputs) and less power needed/required OR less injection and more power requirements in hardware. Personally, I am good with the second option and an extra psu or possibly 2 for the show will only run me ~$24 if I do a rainproof psu during the pre-sale. I can tuck maybe 2 of those psus at the base of the tree and use them to cover the tree and the surrounding items.
An interesting reference pages to this topic, which I just found after writing all this:
Holidaycoro – 5 Volt or 12 Volt – Which Should I Use For My Project
diychristmas.com – Choosing a Pixel Voltage: 5V vs 12V
tl;dr – 12v is my choice.