I'd start this idea at what images you want to create, not at what format you are going to use. Suit the format to your idea. There is almost no reason to shoot sheet film just for a wedding portfolio. There are reasons to shoot it for actual wedding work. However, for what you describe needing, and what you will be able to shoot as a guest, I don't see any real benefit from using large format, but I see a plethora of drawbacks. Do you have a Rollei or a Hassy? If so, I'd use that.
Additionally, professionally speaking, I feel that one should build their portfolio from work shot at weddings at which he or she has been hired to be a photographer, not at another photographer's wedding. The best way to do this as a beginner is to work as a second or third shooter for the main photographer. If you are good, you'll have a portfolio but quick doing this.