I have just bought a Panasonic camcorder capable of recording in formats up to 1920x1080 - 50p. Movie Studio does not seem to have templates for working with this format, so would I be better off sticking to shooting in 1920x1080 - 50i on this camera?
You aren't stuck with using the templates as-is.
Search "Match Media Settings" for the Project, and Customize for the Render.
That's all you have to do.
It depends on your final destination for your video.
Youtube, DVD and most BluRay players will not accept FullHD 50p films anyway.
You can see 50p films on computer, but most portable devices won't allow you to watch it in natural resolution.
You can render your video in movie studio as 50p. For example, render as video for windows - SonyYUV, uncompressed avi, sound as PCM and then use external encoder like Handbrake to encode it to avc format. Intermediate file is going to be big though.
"Youtube, DVD and most BluRay players will not accept FullHD 50p films anyway."
Youtube accepts 50p, and converts it for delivery. PAL DVD and Bluray players should accept 50p, as long as it is flagged 50i for analog delivery, that trick keeping it in compliance with the spec. Similar flagging tricks are used almost universally for NTSC and 24p.
That all being said, OP still has not told us what he intends to use the video output for.