I think 10.7-10.9 are all about the same on that machine so I don't see why not to install 10.9 Mavericks since it is latest and most polished of them with Aqua UI.
Yosemite while having some nice UI enchancement has some quirks regarding redraw speed. And is clearly not as polished as Mavericks.
For great speedup you might consider buying an SSD though. If you don't use Superdrive then you might just replace it with 9.5mm PATA to SATA Optibay and save on SSD size. If you go this way install HDD into Optibay adapter and SSD in place of HDD.