That's odd that she's leaving it up to you to decide which one to take, but anyway..You will most likely lose weight with Topamax. However, the cognitive side effects are sometimes worse with this drug, i.e. memory, concentration, "word-finding". To be honest, Lamictal had those cognitive effects for me, though most people don't report that effect with Lamotrigine. Topamax more effectively treats temporal lobe epilepsy. What kind of seizures do you have?