The term 'cholinergic' is sometimes used to refer to supplements that positively influence acetylcholine (a neurotransmitter that mediates muscle contraction and learning) signalling in the brain. This can refer to compounds that indirectly increase acetylcholine levels (such as inhibitors of the acetylcholinesterase enzyme) or supplements that deliver dietary choline to the body.
Compounds that can increase acetylcholine activity in the brain are referred to as cholinergics, and a subset of these compounds are just those that confer dietary choline
Choline is the basic form, and is usually bought as bitartrate (41% choline by weight; with 1,000mg choline being found in 2,400mg choline bitartrate). This is the simplest and cheapest form of choline supplementation, and while it does produce benefits for the body it may not increase neural (brain) concentrations of choline reliably.
Although basic choline supplementation is likely not effective for nootropic purposes, it is effective for increasing systemic (not neural) choline concentrations and benefitting the liver. Furthermore this form of choline can reliably increase systemic trimethylglycine levels (a metabolite) which is another way it can be beneficial, just not for neurology.
These effects extend to another simple form known as choline citrate, which is a comparable weight of choline.
Although not the most effective nootropic, choline (bitartrate) can be used to aid the liver and is the cheapest form by a large margin. If not seeking out nootropic usage of choline, then this form is likely the most cost-effective
CDP-Choline is a choline and uridine prodrug, and readily dissociates into both free choline and cytidine following oral ingestion (and in humans, cytidine readily converts to uridine); it is 18.5% choline by weight, and 1,000mg of CDP-Choline confers 185mg of choline. CDP-Choline is able to increase neural concentrations to a greater degree than choline itself, and supplementation of CDP-Choline also confers a second supplement with cognitive benefits.
Alpha-GPC is 40% choline by weight (1,000mg of Alpha-GPC confers 400mg choline) and is a prodrug for both choline and glycerophosphate. Although glycerophosphate can also help with production of cellular membranes it is not a very well investigated topic. Alpha-GPC is mostly used due to being an efficient form of choline supplementation (high choline on a weight basis and very readily absorbed and delivered to the brain).
Both Alpha-GPC and CDP-Choline are forms of choline that are prodrugs for two molecules, choline and something else that helps with membrane production. Although more expensive, both are suitable forms for nootropic purposes