What's the difference between a hydrocele and a spermatocele?

Different structures. A hydrocele is a fluid collection around the testicle. A spermatocele is a fluid collection or cyst arising from the epididymis. Both condition are generally benign.
Big difference. A hydrocele is fluid around the testicle. Normally, there is some fluid around the testicle to protect it. Excessive fluid is referred to as a hydrocele which can reach the size of a grapefruit. A spermatocele is usually smaller and is along the path of the epidydimis or vas deferens. Occasionally, they can cause pain. Most are asymptomatic and do not require treatment.