"Beyond the restaurant"
The terms are equal, where choosing to use a-versions means a more radical (rural areas, working class, speaking broadly) bokmål and choosing en-versions is conservative (city dialect, up town, decent/proper speech). If you decide you like bortafor, you also need to choose feminine nouns over masculine (jakke (jacket), [m] en jakke - jakken or [f] ei jakke - jakka. Katt (cat), [f] ei katte - katta, [m] en katt - katten. Stue (livingroom), [m] en stue - stuen, [f] ei stue - stua.) If you look up jakke or stue in my link, you will have both feminine and masculine listed in the same entry. But for katt you need the -e to make it feminine, katt is masculine and katte is feminine, making it two entries in the dictionary (very few people would actually differentiate the gender of the cat by choosing to say either katt or katte)
As an example "The cat is in the livingroom" Katta er i stua - two radical forms Katten er i stua - mixing the two forms, your teacher would make a note on that but it is grammatically correct, just off. Katten er i stuen - two conservative forms