TL;DR - they're both designed for construction purposes and meet standards*
(*in the US, at least)
Generally, Simpson StrongTies are used for all sorts of construction purposes. Read through their documentation to ensure this will meet your needs. Since you don't indicate what you're building, it's really hard for us to say for sure.
The ATP4 is for 4-by material (~100mm) while the ATP6 is for 6-by material (~150mm), so make sure you're getting the correct sizes for your project. Also, be sure to use the proper screws (they specify that it has to be their structural screws and washers, but I'm sure any similarly spec'd structural screw & washer will do the trick - their's will be powder coated to match, which is an advantage) and to fill all the mounting holes in the bracket.
Personally, I'd be a bit concerned by the fact that the ATP4 has only one hole for each piece of wood. In my mind, this makes the cut of each joint extremely important. You'll need to leave some room for expansion (no more than 1/8" ~3mm gap), but too much leaves you with a pivot point on that single screw. I doubt that this would fail any certification process they put them through, but it would make me a bit nervous because I know my ability to get consistent quality joints.
The MiTek is also used for many construction projects. Personally, I'd feel a bit better about this particular strap as it's got 4 nails going into each piece of wood that it's attaching to, so there won't be any pivot issues.
Of course, the MiTek is "ugly" looking galvanized steel while the Simpson is "pretty" looking powder coated (not sure if it's also galvanized - I didn't look for that, but I'd be surprised if it wasn't) steel. On that end you're comparing aesthetics vs price and that's a decision only you can make.
I've used both Simpson & MiTek on projects at my house (a garage, deck and storage shed), but I've never used the decorative ones, as mine have all been buried and not for show. I've been happy with both and my choice was made mainly on what was available on the day I purchased.