I went through a similar predicament with my Vauxhall, and this is what happened. A universal antenna stem broke off a few months after installing it (the original one was basically degraded by weather in the course of the years, normal occurrence for very old cars). Don't know if it hit something or it was just a matter of aerodynamic resistance and poor quality materials. And besides that, it looked awfully short compared to the original. Something comparable to a TETRA (a UHF-based digital two-way radio system used by police and emergency services in Europe) vehicle radio set aerial, rather than the definitely longer bee-sting aerial usually used for FM broadcast. And reception got poorer as a result.
The OEM aerial costed something like 5 times what i paid for the universal stem, but it's longer and sturdier looking, it hasn't broken yet and it's as functional as the original one in terms of reception.
So i recommend you go with OEM. It pays off later.