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Heat mats are a great way to get a jump start on the growing season, especially for those in the northern states where the growing season is very short. Some seeds will germinate in cooler soil but most prefer warmer soil in the early stages, and one of the best ways to increase germination rates is by providing consistent, gentle bottom heat, and that's where heat mats come in.
The heat mat you choose will depend on how many seedlings you plan to grow, how many electrical outlets are available, and what type of light you'll be using. For many gardeners one or two small heat mats are usually sufficient, but one large heat mat is sometimes better than several small ones. The small heat mat is 9" x 19.5" and will heat one standard (10" x 20") seedling tray. The large commercial heat mat is 60" x 21" and fits up to five standard trays. It only needs one electrical outlet, only one thermostat, and is expandable. There's a regular plug on one end, and a strange looking plug on the other end that allows for daisy-chaining up to four more heat mats off the first mat. The commercial heat mat isn't the cheapest option upfront and it doesn't work without a thermostat controller so that needs to be added to the cost of the heat mat.
You might be able to use the small heat mat without a thermostat depending on your home's ambient temperature. The small mat is designed to raise the rooting area temperature approximately 10-20° above the ambient air temperature. So if your house is at 68°, the potting mix will be between 78°-88° which is fine for seed germination, and you can shut the heat mat off at night. If the heat mats (small or large) are going to be placed on a cold surface like concrete, wood, or kitchen countertops, it helps to put some type of insulation such as styrofoam, a piece of carpet, an old blanket, etc., underneath the heat mat to help disperse and retain the heat evenly.
Hydrofarm is a very well-known name in heat mats, for both the small heat mat as well as larger commercial mats. The large commercial mat came from Johnny's Seeds for $129, but Amazon has what looks like a similar commercial mat for (currently) $20 less and that's with free shipping . If you decide to buy the large mat, check that you're buying the regular heat mat first, not the add-on mat. The add-on mats typically don't have the AC plug.