You have a few conflicting issues to way up.
First the extra mana regen you get from a mount is superb and worth every PP you will pay, but it is outdoors only.
Lizard has the big advantage of providing a clearer field of view than a horse (although there are times when you can still target yourself accidentally).
Lizards are very cumbersome, they do not step around very well, take a lot of time to get started and to stop, so when you are trying to loot for instance, you can spend 10 minutes backing up and shuffling around trying to target the corpse. Horses are much more nimble.
At full gallop though lizzies are fine and run up and down hill as easily as running on flat ground. (I believe horses slow down when climbing hills).
The cheapest ones are very slow and are of no use to try to out run any mob that is chasing you.
My preference is for the lizard as it is mana regen that I primarily want and in this case a clear field of view is an important factor.
Celeni has a cheap slow lizard and she simply clicks it off whenever she needs to manouver in any way (she always has SoE up).
The cleric will wait a bit for enough PP and buy an expensive one for both the mana regen and the higher run speed. (Although this is an ego thing, as it would be more effective to simply keep a supply of SoW potions to hand and use the cheapy for mana regen).
Hope this is of some help.