There are dozens of ways to skin this cat. I just want to put this out there though. Tile is a premium product and you should expect to pay a premium price for it to be done right.
Now, there are levels of installation that you can hit. With each level risk is shifted. As a contractor I had to assess the level of risk I was willing to take and what type of business I wanted to run. I will say I only use ditra on floors in my own house and most of the time for paying customers.
If a tile floor fails, there is only one way to fix it and that is a complete tear out. So, just remember that.
Here are your options (some of them anyway).
1. Just thinset tile down with regular thinset. This is still very likely to work well and potentially could last a lifetime if it is done well. You could have some Efflorescence issues if there are water issues under the slab and you haven't done anything to prevent cracking which may or may not ever be an issue. This would be good for a radiant floor.
2. Use a high quality flexible thinset (FlexBond is one example). This is still very likely to work well and potentially could last a lifetime if it is done well. Again, you could still have some issues with Efflorescence but it would do a lot to prevent movement cracking. This will be a slightly less good option for radiant floor. This thinset has more polymers in it so not quite as good heat transfer but I have no idea how much different but probably not enough to worry about.
3. Use a liquid membrane. (You do get what you pay for so remember that). (I'm not saying Redgard is bad but I haven't used it in 20+ years) This will decrease the efflorescence potential and help with cracking. It will also decrease the heat transfer some amount, again, no idea how much but probably not that much in all reality.
4. Crack Isolation membrane or uncoupling membrane (ie. Ditra, Nobleseal, etc). These will do the best job of preventing moisture, efflorescence, and cracking. Hands down the best option. These will be a little worse for heat transfer but I think that talk is overstating an issue.
I really don't think any of these would negatively impact radiant heating in any way. I have haven't seen any data to show that but that doesn't mean it doesn't exist. I would also push for something to help with movement more with radiant since it will be heating and cooling with causes the tile to expand and contract. Tile and concrete don't expand and contract at the same rate.
Tile and thinset are both pretty good conductors of heat and will absorb and release heat similar to how concrete does. It is not going to act as an insulated layer.
Now, I am not a guy who would ever tile his garage but if this were a kitchen on a slab I can guarantee you I would be pushing ditra, and honoring control joints, and making sure I had proper movement joints, and left my perimeter clear. Is a garage going to take more or less punishment than a kitchen? Only you can decide what makes sense for you and and your situation. I will also say I am not aware of any floor I properly installed with a good quality thinset straight to concrete that failed. If budget really is an issue I would push towards Option 2 and make sure the install is spot on.
Also, good thinset will stick to trowel finished concrete better than it will stick to porcelain tile.