I would use the dado blade since you have one. It is cheaper to use it than to replace router bits. Plus it is sometime easier to get a good fit with today's plywood sizes. IMO
Sorry, I was moved by your post and had to comment. I do a lot of dado (grooves) and spline work... ^^^The initial investment is not cheaper.
Someone that has practice with both- The differences between doing dado with a router or a saw is just... well, different.
If I'm doing one or two dado's, I pull out one of my dado jigs and use a router. I use a bit about 75% diameter of the finished size's groove width. You ask why? Because then I can make 2 passes, one each direction of travel, so I can get a snug fit on the groove for the spline joint. It takes less time to setup for a router cut groove than a TS, but the machining takes longer than a dado blade on a TS.
If I'm going to do many grooves, I use my TS. Like I said, It takes just a little longer to set up (not by much time at all). If you haven't set up a stack before... You use different thickness blades and shims together in a stack to get your thickness. After you do it a while, you get a feel for what to do. If not, use a calculator or a chart. Like I said, maybe a bit longer to get the stack together. Machining is a lot quicker than a router. Moving from one to the next cut is quicker...
But doing dado's on a TS assumes that you have a TS with an arbor that can handle the extra width of a dado stack... and the HP to turn it. I have commercial finish quality stacks, as well as commercial standard stacks. I run 4.5 HP w/ 8" and 10" dado's. I would say minimum 1-1/2HP with a 6" dado.
Saying that one is better than the other doesn't wash with me... they are just "different." The width of both can be tailored and tweaked to what you need to do. Someone on a budget can get off cheaper with a router, but it takes longer. Cutting edges eventually wear and need to be sharpened on both, but you get more board feet of cut from a stack. But by the time you figure all the factors in, Dado stacks and the level of TS to drive them is a higher investment.
A low-dollar, usable standard stack can be had for about $50-$70. I'm saying that you can get a stack for about that price that will have a good edge on the sides of the groove, without excessive tear-out... and get a straight cut and even width. That is what is visual in spline joint joining. The quality of the bottom of the groove is not that important until you get to higher end joining.
Besides, the finish quality stacks I have, most people could not afford them and would be prohibitive except for those that really needed them. But for what I do with veneered stock and melamine's, etc... Finish stack and dado scoring (shimmed scoring blades) are a real plus. Which would be a challenge with a router, on the same stock., in the same time and effort.
Not saying that routers don't do a go job, because overall, someone just starting out could, They do a quality job on grooving with a jig. You can get a very quality groove with a router, at nominal costs. One thing you have to take care with is to ensure consistent width of the groove... Then you're golden with that. If not, that will be visual (especially on long grooves, in the joint).
So, yes. There is pluses and drawbacks on both sides. They sort of wash each other out, but basically-- i see my decisions on those being the application and volume.
Next is a question--
Is your Miter Saw a SCMS? If so, then why now do it with that? If it's already set and there, in a pinch, I do up to about 14" long grooves with mine. Gives someone practice in that technique. (Using 1/8" to 1/4" FTG tipped blades)