Yes, I'd use Dowflake over Prestone Driveway Heat. Changing suppliers is not the only risk, as I've not assayed Peladow (the material in Prestone Driveway heat), but many folks have used it and I've only seen one reported problem that may not have been actually related.
This may help:
An Improved Do-it-Yourself Two-Part Calcium and Alkalinity Supplement System
http://reefkeeping.com/issues/2006-02/rhf/index.php
from it:
Substitutes for Dowflake Calcium Chloride
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If Dowflake calcium chloride or a repackaged version (such as All-Clear) cannot be located, Peladow or Dow Mini-Pellets, which are dehydrated versions of Dowflake (that is, they have less water in the crystals), can be substituted. In addition to the Peladow brand name, Peladow also is sold as Prestone Driveway Heat and possibly as some other common brands. Kent Turbo Calcium is also suitable and is an anhydrous calcium chloride. Any FCC (food), USP (pharmaceutical) or BP (pharmaceutical) grades of calcium chloride should be suitable.
Peladow, Dow mini-Pellets, Prestone Driveway Heat, Kent Turbo Calcium and other dehydrated calcium chloride products are more potent than Dowflake. The dehydration makes them both more potent by weight, and more dense, so they are much more potent by volume. The problem is that it is rarely clear how much moisture they contain. Peladow specifies 90% calcium chloride minimum, but it may be higher in some cases. Dow Mini-Pellets say 94% minimum, but it actually has a lower bulk density than Peladow. The best guess for an amount to use is based on the hydration levels and bulk density provided by Dow for these products. Using these numbers, I suggest that aquarists use 20% less VOLUME of the dehydrated versions in the recipes than the Dowflake they call for. So a recipe calling for 5 cups of Dowflake would use 4 cups of Peladow, Prestone Driveway Heat, Kent Turbo Calcium, etc.
Choosing other unknown brands of any of the products may be fine, or not. I've not tested them for purity.