What is the difference between a scope and a red dot sight, and what should I buy?

A scope magnifies the image, useful for long distances—ideal for snipers. A red dot sight does not magnify but allows fast aiming at short and medium distances. For assault replicas, a red dot sight is recommended; for sniper replicas, a scope is preferred.

Selection of optics should be matched to the replica type and play style.

For M4 / AR-15 replicas, which typically have RIS rails, lightweight compact red-dot sights work well, for example:
https://www.taiwangun.com/red-dot-sights/reflex-sight-with-real-fiber-optic-black-pcs
https://www.taiwangun.com/red-dot-sights/standard-cqb-red-dot-sight-with-cantilever-mount-black-pcs
https://www.taiwangun.com/red-dot-sights/maverick-ii-plus-1x22-dbr-red-dot-sight-black-vector-optics.

If you want greater versatility you can pick a red-dot with a 3x magnifier; an example magnifier:
 https://www.taiwangun.com/scopes/maverick-3x26-magnifier-vector-optics. It is recommended to contact customer service to check compatibility between the magnifier and the chosen red-dot.

On AK replicas the mounting type is critical — many models require a dovetail side mount or an adapter to a RIS rail. A railed scope mount for AK (M-METAL) that allows mounting optics on a 22 mm RIS rail can be used: Railed Scope Mount for AK: https://www.taiwangun.com/mount-rail/railed-scope-mount-for-ak-black-m-etal.
This makes it possible to fit sights such as the 1P87-style Russian optic, for example:
 https://www.taiwangun.com/red-dot-sights/airsoft-1p87-style-russian-optic-sight-for-ak-ak12-aksu-jj-airsoft.
Check whether your replica has the left-side rail on the receiver cover (visible in product photos) before buying. Example of AK replia with left-siderail:
 https://www.taiwangun.com/assault-rifle-aeg/cm-056m-full-steel-ak-replica-eshooter-etu-cyma

For sniper replicas such as M62, CM.707-U, MB08AE or M98B, precision and magnification matter. Example scopes are:
 https://www.taiwangun.com/scopes/3-9x40e-rifle-scope-black-pcs and the https://www.taiwangun.com/scopes/luneta-victoptics-s6-1-6x24-vector-optics
 These scopes offer decent image quality and parallax adjustment; mounting is usually with 25/30 mm rings onto a RIS rail.

For CQB and SMG replicas (for example M917G UTR45 or WE06), lightweight open reflex sights are best, such as:
 https://www.taiwangun.com/red-dot-sights/mini-pistol-carabine-red-dot-sight-black-jj-airsoft
 https://www.taiwangun.com/red-dot-sights/hd118-electro-dot-sight-with-quick-detach-mount-black-pcs
 which provide a wide field of view. Mounting is on a standard 22 mm RIS rail.

In summary: a red-dot (collimator) gives fast, intuitive target acquisition and is best for assault replicas and CQB; a scope (optic) gives distance precision and is suited for sniper replicas. Choose optics according to replica type, play style and mount type. Taiwangun’s English site has a wide selection of scopes and red-dot sights from brands such as JJ Airsoft, PCS and Vector Optics