Rasgulla is made from chenna and sugar syrup and is spongier. Rasmalai is a creamy version of rasgulla served with a creamy rabdi base.
Rasgulla is soaked in sugar syrup and is pure cheena so its spongy, rasmalai gets a creamy texture because of rabdi.
Rasgulla are made into small round balls shape whereas rasmalai as an oblong eclipse shape.
Rasgulla is primarily sweet infused with sugar syrup whereas rasmalai have a more complex flavor with all the spices in rabdi.