An angle grinder can use several "disks" that are very efficient for removing wood. You can buy what is essentially a chainsaw wrapped around a disk. Great for carving and able to throw chips around like mad, but I would NEVER bring it anywhere near a possible hit with a nail.
There are also textured carbide wheels you can put in a grinder. Also able to grind wood rapidly, and perhaps safer if it hits a nail head. Finally, you can buy a disk that is essentially layers of overlapping very coarse sandpaper. This will be less aggressive yet and the paper will wear down with use.
The other problem with using an angle grinder is it won't leave you with a flat surface easily. A grinder is a very local tool. And you want a reasonably flat surface. So if you did choose to use an angle grinder, you might still want to come back to it with a belt or disk sander to get it flat.
Of course, it is also true that other tools may not like nails either. A router for example is a BAD idea here, as that rapidly spinning carbide bit may well meet an untimely end as soon as it hits a nail. And there WILL be nails in a subfloor. If this is a sufficiently old house, they may even be great big, clunky square headed nails.
A problem with anything you use that will remove a lot of wood is the dust it creates. So make sure you use dust protection for the well being of your lungs.
Overall, my choice would be a belt or disk or drum sander designed for floor use. They are fast and efficient, and will give you a flat floor. And they will grind off any nails they hit, although nails may wear down the belts a bit more than just wood.
In most cases the cost is directly related to expected durability. In expensive commercial/professional grade tools, parts spin on ball bearings and motors are more powerful. They are designed to run allday every day at maximum speed where time is money. Home owner grade tools typically are designed to be used for brief periods of time and at less than maximum performance levels for the majority of their use. They contain less powerful motors and parts spin on bushings instead of bearings. In most cases they will last a long time as long as you respect the design limitations of the tool. If it starts to get warm while using it stop work and let it cool a while. Don't force the tool if it starts to bog down back off the pressure and let the tool do the work. I have a 10 year old $30 grinder that I cut over 100 slate floor tiles with and it is still running strong.
Best Answer
Electric angle grinders are very useful tools, they can also be extremely dangerous. First and foremost, select a tool with as many of the latest safety features as you can afford:
Buy gloves, goggles and dust mask when you buy the tool
Amps indicate the power that the motor draws during use and generally indicates the size and power of the motor (higher is "better"). I personally think a well built tool with the features I want and which is comfortable for me to hold and use is more important than the amp rating. If you are routinely grinding and cutting thick heavy hard metal materials you might consider a bigger/more powerful machine.
Voltage should be whatever your household current is (220-240vt angle grinder would not be practical for home or general shop use, pretty much industrial only)