This is a crude measure allright, apples and oranges (or should I say biceps and quadriceps). I mean one picture might be worth a sousand words, but for sure one squat is not equal ten arm curls... Would be nice to have a calculator that will take into account the different exercises and muscle groups and give a total grade to the workout effort. Since ExRx has that wide collection of both, maybe they can develop such a calculator (i couldn't find one in the "calculators" section. BTW, seems there is much to improve there, but that's another topic).
You've missed my meaning here. No one compares work capacity on curls to work capacity on squats! That would be silly. You can discuss the work you do on squats today with the work you do on squats next week, or the work you do on curls now with what you did when you were 17, but why would you ever compare squats to curls?
My point is that you have to think about your work capacity. A program that has you doing 5x5x100 pounds (on whatever lift) probably will be much harder for you to sustain than one that has you doing 3x4x120 (on tht same lift). That's 2500 pounds of work compared to 1440. You'd probably increase strength better, and be less likely to have issues with fatigue with the second program. But work capacity can be trained. Don't try to jump from doing 1400 pounds of work to 2500 overnight. Far better to add work slowly. That is often not considered in planning a program.
You say you can't work at anything near your 1RM, but I say it's all in how you do it. You won't gain much strength if you don't do some lifting in the upper end of your capacity. When the weight gets heavy is when you have to pay attention to the the total work. You said that you can't do 15 sets of 2 as you can't go anywhere near your 1 RM. If that's how you try going near your 1RM, then well, of course! 15 sets of 2 would be brutal for a very experienced lifter. But if you are doing 3 sets of 4 relatively-heavy reps, there is room to gradually increase. Add a rep per session for a while. Then try 4 sets of 4. As the volume gradually builds up, then drop the rep count back down but with a bit more weight and continue the process. All the options you mention include a large total number of reps, so the total work is going to be very high. Many programs are build on the use of Prilipin's table (no, I'm not going to look up the correct spelling) which was from descriptive research done on elite Russian Olympic weightlifters. That's what many people use to design programs for middle-aged men trying to get a little stronger. The research just doesn't apply. You are talking about 36 reps. You probably need to be thinking about 10 or 12 reps at your working weights.
What I was saying about emphasis is that you can't expect to make good progress if you give equal emphasis to every exercise. You say that you do 4 muscle groups (more on this concept in a minute) with up to 3 exercises per muscle group. The only way that you can sustain this is if you don't work very hard at any one lift! You have to give mediocre effort to each lift, and your results will be mediocre. If you want to make good progress, you have to decide what's important to you. You can't imagine that a curl and a squat are of equal value, can you? You need to both decide what's important overall (lift-time perspective) and what you want to emphasize right now (a 1-month to 1-year perspective). Then structure your workout accordingly. If the squat is more important than the curl, then don't spend equal time on them. Don't put equal energy into them. Your high-priority exercises should be given more time and effort. They should be done when you are the most rested in your work-out week, and at the beginning of your workout. As I mentioned, I currently have one lifting day when I only do one exercise. It's the lift that I have decided is my top priority for the next 6 months to a year. Not for the rest of my life, but I want to make progress on this, so I don't want to dilute my effort much. There are 2 other lifts that are "tier-2" lifts for me, and I do them together on the second day of my training week. 3 other lifts are important to me, but I'm willing to allow them to simmer on the back burner for now.Here's a link that might be helpful on this topic.
Why do you need 3 exercises per muscle group? Yeah, they can have slightly different emphasis, but not THAT different. If you try to think about muscles or even muscle groups, there is an overwhelming number of exercises you will feel that you "need" to be doing. Think movement. That's what your body does. Although you can quibble, that can be reduced pretty well to 6 major movements: vertical push (think press), vertical pull (think chinups), horizontal push (think bench), horizontal pull (think row), quad-dominant legs (think squat) and posterior chain dominant (think deadlift). Not the exercises I mentioned aren't the only possibility for each movement. There are many good ones in almost all categories.
What Stu said about machine vs. free-weight vs. body-weight exercises is very important, and I agree with him enthusiastically. Instead of deciding if machines or free weights are better, you need to be doing good, safe body-weight work. You are worrying about the difference between an MBS and an MA, when you haven't started college yet. You can be working on body-weight squats, several varieties of single-leg work, push-ups, pull-ups, horizontal rows, etc. As you gain strength at those, then start to include a little added weight to those. Only when you can't practically add weight to those should you graduate to dumbbells and then to a barbell. Body-weight work will build your stabilizers, and your endurance and your work capacity.