The answer is..... it depends on what you are doing with the tank
Some corals will love lots of flow (SPS for example), others will hate it (a lot of LPS corals for example). Same for other livestock. However depending on how you tank is laid out and your powerheads are positioned you can normally create a pretty varied mix of high flow and low flow zones to suit most things.
To be honest I have been out of the loop in marine stuff for about the last 12 months. All I have at the moment is a big tank storing my LR and a few soft corals so there might be new kit on the market since I last really looked.
Good brands are Tunze and Hydor Koralia's. Vortex systems 9MP10, MP20 and MP40) are amazing but cost a fortune.
Personally I have always used Koralia's. You can get them second hand for a good price and they are pretty much bullet proof (make sure you get a few spare shafts in though as they tend to be ceramic and snap easily if you are not careful when cleaning).
Regarding the cheaper make powerheads. They have their pro's and cons. Sunsun was fairly common awhile back (and might still be). Generally with the cheaper powerheads:
The don't give as much flow as they advertise.
You can't get replacement parts (though as they are cheap its not such an issue to just replace rather than repair).
They may be quite loud.
"Extras" such as magnetic mounts are not there (so you have to use dodgy suction cups).
Generally you get what you pay for. I personally would prefer to go for a second hand Koralia over a cheaper ebay pump but that's just because I know its reliable and I know from personal experience how much it sucks to lose months of work and hundreds or pounds of livestock from equipment malfunctions.
All that being said though I'm looking to replace one of my Koralia 4's that I melted (took it out of the tank and plugged it back in by mistake so it was running for about 2 days out of water.. it didn't end well). I'm actually considering getting a couple of cheap pumps off ebay to get my by for the time being but firstly I will be looking for second hand Koralia's and Tunze pumps.
With regards to 1 big one vs multiple small ones. I would always go for more than one power head. It gives you more options for setting up as you can control where the high flow and low flow parts are. It also helps to stop you getting dead zones where there is no flow. Lastly if you only have 1 pump and that pump fails then you have a serious problem and very little time to resolve it (i.e. you have about a day to get a new pump in there before things start going down hill). With multiple pumps if one pump dies its still a pain but it wont wipe your tank out. Also as they are small pumps they are probably going to be cheaper and easier to replace than 1 big pump. Only exception I would give to this would be the Vortex pumps. Even then though I would have a couple of spare powerheads sitting around "just in case".
Hope that helps a bit.
Edit: One other thing. Unless you are getting a huge RO unit dont expect it to fill a 100g ish tank over 2 days. If you run it right into the tank and leave it on 24/7 it might get close but in practice you are probably looking at quite a bit of time to make that much RO water.
