I think oil pan/oil heaters are preferable to coolant heaters. Your standard block heater or coolant heater will help the engine start but even if your block is warm, your oil has spent the last however many hours sitting in the bottom of your oil pan with no contact with the block. Coolant heaters help the engine get to operating temperature quicker and help operator comfort, but the initial start will still be with cold, thick oil. The best engine heating systems (such as Kim Hotstart, etc.) all provide means of heating engine oil. Warm oil also does more to reduce wear as it gets through the engine much quicker. I know on my 86 Golf 1.6 NA I haven't noticed any difference in cold starting with my block heater (coolant only) on or off. When I do use it I leave it plugged in from whenever I get home from work until I leave for work in the morning. The only difference I notice is quicker windshield defrosting and less time with the cold start pulled out. Actually there is a shorter period of time for the glow plugs to cycle, but the actual cranking time once the glow plug light goes out isn't noticeably different. The last week or two I haven't even bothered to plug it in.
