Perhaps the biggest deciding factor is often what a particular shop is already using or familiar with. Most will continue to use what they know. Further, if a shop changes their primary routing protocol, they often lack experience to do it "right", especially large scale, since they don't really know what they don't know.
When trying to decide which to use, above and beyond the forgoing, it really depends, firstly, whether you want to be "locked in" to using Cisco devices, as EIGRP is, or was (Cisco has released some of EIGRP for other vendor usage, but I believe not all of it), proprietary. Not using EIGRP does put you into a much better bargaining position.
As to scalability, not really a huge difference, just how you do it varies between the two. With OSPF you use areas and with EIGRP you (as I understand it) use summarization.
As to convergence speed EIGRP has a slight edge over OSPF, but both can be configured/tuned (at least on Cisco devices) to generally obtain subsecond convergence.
Also on the subject of Cisco devices, Cisco's OSPF implementation is often much (much) better than many other Brand X vendors. (When you can use Brand X for half the price, different parties will often want you to do so. However, you'll may then discover why Brand X is half the price.)
