I'm not sure at what point a plate or counterflow becomes the better option ... if you're talking 15 gallon batches you may want the plate chiller (or counterflow ... i've never used counterflow). I know for our 35+ gallons it's the only option. On our beers we can chill 37 gallons of beer down to pitch temp in about 17 minutes using under 35 gallons of water ... while on some of our super hoppy beers (DIPA, IPA, etc,) it's more like 32 gallons of beer in 25+ minutes (beers with over a KG of flameout hops) ... the thing i hate about a plate chiller is that the hops slow it down considerably and we have clogged it before, which can create some chaos in the brewery. We use the duda diesel plate chiller and we're happy with it - i can send you the model number if you'd like ... but we literally spend an additional 20 minutes at end of brew day cleaning/recirculating through it, which probably wont change whether you're doing 5 gallons vs 37 gallons. To me, if you can get away with a 50 or 75ft immersion chiller, i would personally go this route.