AIs not only prevent sides but they prevent the free testosterone in your system from being converted and wasted into estrogen. SERMs are receptor blockers but DO NOTHING to "keep estrogen low during natural testosterone recovery (PCT)".
For more detail: AIs are taken to stop the enzyme from creating estrogen or to stop the action of estrogen on the receptor (note this is not the same as blocking the receptor like nolva does). They help treat sides such as high BP, high estrogen, water weight/bloat, oily skin, etc. to be used on cycle leading up to post cycle therapy (pct).
SERMS:
Nolva is a mixed receptor agonist/antagonist. It is an antagonist in breast tissue where it blocks the estrogen receptor from binding with the aromatase enzyme. In other tissues its an agonist blocker meaning it does react biologically with the tissue. Nolva is used in PCT to help block the receptors from accepting estrogen during the estrogen rebound when coming off cycle.
Clomid is another serm that blocks estrogen from binding to the hypothalamus. It inhibits the negative feedback loop of estrogen on gonadotropin release and By doing so it up regulates the HPTA.