Immersion.
I've never heard of anyone using a gravity fed plate chiller, but, especially with 80 degree source water, you won't get down anywhere near pitching temp in one pass, so you will need to circulate it, and a pump is going to be the only practical way.
Immersion has many other advantages; it is just simpler, easier to clean, easier to sanitize, easier to set up and take down, etc, etc.
If your kettle is oversize and your wort level is low, you probably only "need" a 25ft, but, I would still get a 50' for future proofing. Ever since I got a 50, my 25 has sat in the closet. I might use the 25' as a prechiller one day.
Either way, with 80 degree source water, you will need a really good prechiller setup to get to pitching temp, or you'll need to finish chilling in a fermentation chamber as I do. Or an ice bath.