The plate chillers are going to be more efficient than tube-in-tube counterflow chillers in general. The drawbacks are:
1) Plate chillers can be pricey, but the performance seems to be worth it from all the folks I've talked to.
2) Plate chillers are susceptible to clogging with debris such as pellet hops.
Depending on your climate and the kind of beer you brew, the extra performance of a plate chiller may not even be necessary. In other words, if your tap water is 60 F for most of the year and you brew ales, a counter flow will probably do you just fine. However, any chiller is going to need help once your chill water starts to creep up over 70 F. Here in Knoxville, my chill water is about 78 - 80 F during the summer so summer brewing is a bit of a challenge.
Prosit!