NO both are fine , depends on the quantity you can buy and wether you can use it all before it is out of date ,both can be used to kill maggots and crovect protects against blowfly BUT spot -on does not , both kill ticks . Spot-on is easier to apply just a small amount on the skin on the shoulder then it is absorbed into the skin and moves slowly around the body , so takes a while to kill . Crovect is a large amount in a pin stripe from head to tail and then travels around the body via the lanolin , it works best if you can part the fleece and get low down the wool near the skin , often hear that it doesn't work well at this time of year on very heavy fleeces but with only a few to do you can take time to part each fleece and get it down to the skin