Hello Amy! They are a litle bit different.
When you use "À FRENTE", you give the idea of sequence/disposition/order. For example, in a line at the supermarket: who is before me is "À FRENTE" (or "À MINHA FRENTE"). If there is someone after me, I can say "ESTOU À FRENTE DE UMA PESSOA (QUE ESTÁ ATRÁS DE MIM)."
When you use 'EM FRENTE", you give the idea of a simple relationship in space, without associating the notion of order/sequence. For example, I can say that museum is right in front of the garden, so I use "O MUSEU FICA EM FRENTE DO JARDIM".