Dunno, both have a good reputation. I've had nothing but Escorts over the past 20 years ( still have my first 'big box' Escort and the little Passport, why I don't know... ), and have yet to get a ticket while they've been on. Currently I run their 2-3 year old Solo cordless. Saved my bacon just this AM on the way to work. And countless times over the years.
I see the ads in AutoWeek all the time for the Heart unit. Bottomline, if the box is yelln' your going to slow down until it is quite. I don't really care to know that there are 3 'bogies' ahead or one. One is all it takes. At $300 plus, they seem mighty proud of that receiver.
Now, that said, cheap units like Cobras etc. , will drive you nuts with false alarms. Either will do the job, I'd base my final on price.
my 20 years worth of . 02