Well, I personally do not like to use a slann for many reasons, but that's just me. For an unbiased opinion I will give you this:
Slann are primarily used to give a HUGE magic boost to your entire army, they can play multiple roles, since they can be both BSB and General, but can cost a lot of points as well. They can absolutely dominate any magic phase depending on the upgrades they have, and against DE they can help stop some of those small pesky little spells they do to bait you. Another cost to factor in is the Temple Guard unit you should bunker him in. DE aare fast enough to get something to a lone slann to kill him, so a TG unit would be a good addition. Most people normally would not take TG without a slann so you are adding even more points to your slann choice in that respect. But good thing is, it will take ALOT to kill off that unit.
Oldbloods in my opinion are not really worth taking without a carnosaur, so already they cost 355 points which is expensive. After upgrades you are looking at something in the 400+ points range. The slann is expensive too, but slann can deal with multiple units and OB cannot. The best an OB can do is beat up on one unit (hopefully not a tarpit) for a turn or 2 and move to the next one. The good part about the OB is that you have a strong General that can take on almost any adversary, and he is an absolute beast in combat. It would be wise to have a couple of skink priests around or engine of the gods to help with magic boosts.
I personally use a OB in my larger point lists (2500+) and just hero choices for anything under. However, the most common response you will get is to use a slann.