I'd go with the SR5 cluster if you can find one - now and then they'll pop up on ebay for a reasonable price, keep an eye on the classifieds in the various Toyota forums as well. I got my last one 3 years ago for $70 and thought that was a good price.
All you need to swap is the oil pressure sending unit - the original is just on/off for the idiot light, the SR5 sending unit is for the gauge. Also, 6 cyl or 4 cyl doesn't make a difference, as long as you're not afraid to take it apart a bit there is an adjsutment you can make for it to work with either.