I won't get into the air vs mechanical debate.

How "strong" a set of retracts are basically depend on 2 things.
1. How well are they secured in the wing or fuselage.
2. What size is the strut ?

If they are secured PROPERLY and REENFORCED properly, both are strong. The strut size is another matter. The MAJORITY of 40 - 60 size retracts have a 5/32" strut. My experience has indicated that this size strut can bend fairly easy, especially if flying off of grass.

I strongly suggest using 3/16" diameter struts. They rarely bend when flying off of grass. Some manufacturers now offer retracts with this size strut, and others can be modified to accept this size strut simply by drilling. Robart has both mechanical and air retracts with 3/16" struts. Spring Air has air retracts with the 3/16" strut.

A couple of suggestions/hints:

Instead of bending the strut to make the axle, pick up a set of DuBro "EZ-Axles". You cut the strut off at the length you want, file a flat spot, and put these on. Make sure you use some locktite on the screws. This eliminates one leg being longer than the other and the aggrevation of retempering the bend you heated up.

For mechanical retracts you want to have as straight a connection from the servo to the retract as possible. MAKE SURE THERE IS NO BINDING. With air retracts make sure to use quick disconnects (makes life MUCH easier ) and route the lines so there are no kinks and wear/abrasion areas are padded.

For attaching the gear doors to the struts SIG has landing gear hold downs. They are nylon preformed straps that come in various wire sizes. I use a piece of 1/4" square basswood between the door and the strut to attach the gear hold downs so I have adequate space/holding for the attaching screws.

Hope this is of some help to you.