Block heaters have been installed from that factory in frost plug locations for years and the rarely ever cause any problems. In my area all new cars sold (at least domestics) are equipped with block heaters when new (some Japanese makes still have them as an option). Even on the rare occasion that a block heater does leak, I never seen one that was a catastrophic leak. Usually it's minor, so much so that it often goes unnoticed be the customer. A tank heater is more likely to cause leaks, since it introduces more hoses and connections to you coolant system.
That said, my former daily driver, a 1985 Olds Delta 88 with an all cast iron 307 V8, was equipped with a Zero Start 1500 Watt tank heater. It worked far better than a block heater for prewarming the engine. That old cast iron engine took a long time to heat up, but with the tank heater it would be blowing heat after idling for 5 minutes in the driveway when -30.
The tank heater is definitely a worth while option, although a 1500 watt unit would be overkill for a 4cyl with a small cooling system (there are several sizes available). Just keep in mind, it may not work well with your application. I wanted to install one in my truck, but it would work due to the block drain location and the heater hose location. These heaters must be installed at the lowest point of your cooling system to work properly (thermal siphoning). This was easy in the big engine bay of my old V8 Olds, it may not be possible in your car.
I am not sure how harsh WI winters are, but personally, I think your best bet would be to install the block heater. Synthetic oil works well for easy starts, but your car doesn't warm up faster like it will with a block heater. Plus, it improves cold start emissions and fuel economy. And not to say you can't use both, I mean after all we are on BITOG, and are a just a tad pickier than the average owner.
If you don't want to spend the cash on the block heater install, go the tank style would be my second choice, simply because it is probably overkill for your application. Than again, you may like overkill, I do.
The pan heater would definitely be the third choice. It works well, but it is the least effective of the bunch when used alone. Like I said, it works well to heat the oil, which is great for reducing cold start wear, but does little to preheat the engine, and reduce engine warm-up time.