Eric - you'd know the difference if you used one first and then the other.
The cheaper shears (I started out with a $30 pair from Sally's) are sharp the first 1-3 times you use them on the ears.... and then they start chewing when you want them to cut.
You don't want your thinning shears to chew at the fur.
This is true of straight edge shears as well, but if you go to cut hair and it is folded the slightest bit - that' a dull pair of shears.
And $120 is drop in the bucket.
A lot of people I know who are grooming multiple dogs as pro handlers - they are using thinning shears that cost $300-500. Just putting it in perspective.
My thing is thinning shears for what you want to use them for (invisible cuts) - you want the best pair you can get to avoid raggedy or frizzy ends left by dull shears.
And it costs $20 or so to sharpen your shears so the longest you can go without getting them sharpened, the BETTER.
I got the above shears at least 5 years ago. And using them as a finisher vs a workhorse, never trimming a dirty coat, etc... - I've never had to have them sharpened. And they are still cutting very sharply like the day I bought them.
The other shears, I'd save money on and get what is practical.
My go to grooming set =
$120 - 6" thinning shears
$60 - 7.5" Dubl Duck Mercedes
$9 - 6" Conair yellow handle shears
$16 - Hauptner real knife (fine)
$40 - Mars Coat King (20-26 blades)
The cheap shears are what I use to clean up feet. In a pinch, that's all I need to use on feet because they are sharp and my dogs have big feet that don't need too much creative shaping.
The Mercedes shears are only used on the edge of the ear.
The thinning shears and stripper are used to clean up the shag on and around the ears.