Ahh, the old, "I'm the boss, but I want to be your friend" situation.
I will tell you to answer your question directly without context, and then try with context.
Namely, be the boss and not their friendly coworker. Tell them this is the job you have for them, and that's it. They are paid to do what you say and be done with it.
But own up to your foreseeable failures.
Now with some discussion.
It is certainly difficult to work on projects at the university when dealing with all the hubub, students, and coworkers sometimes.
I feel this is because of a general "us against them" attitude. Namely, the Phd's against the students, and the technicians against the Phd's, etc., etc.
What also isn't helpful is the general relaxed work life at the university...there isn't the same kind of time schedule pressure like there is in the industry...until there is.
I have been in this exact situation, however on the side being the technician. Constantly arguing over things to do, mostly because my boss had no idea what or how to achieve any given task, except for the theory they learned along their educational path. While I have spent close to a decade working with a trade before entering academics...
Both of us knew what we were doing. One of us had experience making them in real life, while the other had experience doing the maths (to generalize the situation) of it....so as frustrating as it is to be on your side. It's equally as frustrating being on their side listening to someone go on about something they themselves couldn't bring to reality.
There is something I want to point out that is important that isn't clear among academics. There is an entire science to making, and it often doesn't jive with theory.
A well trained technician or tradesperson should have learned along their studies how to predict failure. I find this is often lacking in academics..."if we do this, the math says it'll work"
To quote Adam Savage:
The only difference between a master tradesperson and a novice, is the
novice has yet to learn when something is going to fail, how to
avoid it, and if need be, how to cheat to cover it up.
Your technician when given a task, should be trying to see what happens after said task is done. This is going to naturally cause a discussion. One you seem to not want to have...but you honest to god just have to suck it up and deal with it.
I am a professional in what I do. If I'm tasked with something, it's going to be clearly defined, and it will be completed to that exact specification given. But if your specifications are just garbage, or don't make sense to me for related tasks down the road related to the project, I will want to know why you reason it must be done that way, and I'm going to not want to do it, because it's going to cause problems for me later.
I will argue that it's stupid if that doesn't make sense in the long term. A tradesperson who doesn't care, or bother finding out the whole picture isn't in my opinion worth their salt.
A professional who knows their stuff I trust to tell me when I'm requesting nonsense, and this is something you simply have to learn to work with.
There are boundaries to this. Depending on your assignment, if it's simply making something quick, or something is unimportant, well you can simply tell your coworker, you're the boss and you want this done, no discussion.
If your coworker does this with all tasks, it may not be a sign of professional wisdom, but laziness...and laziness shouldn't be tolerated and this requires being a firm, foot down kind of boss.
But if these discussions are legitimate from their side. Well, there is nothing to do but learn to discuss, or back up with clear, thought-out reasonings why this task needs to be done and be prepared to defend it.
Regardless of what happens, if you do make a decision and your technician said it was going to fail, and it does. You have to own up to it.