First and foremost, which one calls to you more? Sounds like you've wanted a Mini-14 for a while, and it's the one you really want. Get the rifle that calls to you, and you will never regret your purchase.
While I love my M&P 15-Sport, I do not think it's the end-all-be-all of rifles chambered in .223.
I was in the same situation as you. Until the advent of the entry level priced AR's, the Ruger Mini-14 was the affordable rifle chambered in .223.
2012 MSRP Mini-14 Ranch : $881
2012 MSRP S&W M&P 15-Sport: $739
I just happened to want an AR more than a Mini-14. I was able to acquire a 15-Sport for less than a Mini-14.
I definitely love the classic looks of a Ruger Mini-14 Ranch with a wood stock. There is just something about a wood stock that feels right. I'm a child of the 80's and the A-Team allure is strong.
I believe the improvements in the Mini-14 come from a heavier barrel & completely retooled manufacturing machinery. Tolerances are tighter.
1. The M&P 15-Sport is covered by an expressed limited lifetime warranty for the original owner. Anything breaks, call S&W, get a call tag, and send it back.
2. The civilian AR-15 platform has been around since 1963. The AR platform has received updates over the years to make it more reliable.
The Mini-14 is a gas-piston system. The AR is a direct impingement system. They both will get fouling, just in different places.
My 15-Sport cycles any ammo I've fed it from premium Hornady V-Max .223 to Russian steel case TulAmmo .223. It functions with the Federal 5.56 NATO I've fed it.
The gem of the 15-Sport is the 1:8 progressive,
5R rifling. It gives me the widest option of projectile shape to use.
(It's the projectile's shape that determines which twist rate to use, not projectile weight. It just happens to be that boat tail projectiles are by nature heavier.)
This means that I can shoot the cheapest practice ammo found at my local Wally-World.
The 15-Sport is a completely different platform. There is a thread here where someone got a 1"-2" grouping from a rest @ 100yds.
Again, the 1:8 5R rifled barrel is the under rated gem of the 15-Sport.
If you read about mil-spec rifles, you'll read that they're chrome-lined. The chrome lining adds harness and corrosion resistance for barrel longevity. The issue is that adding a layer onto a barrel's rifling decreases accuracy.
If you read about match/target AR's they use a non-chrome lined barrel. No extra layer over the barrel's rifling lends itself to accuracy, but at the cost of corrosion resistance and barrel longevity.
The 15-Sport's barrel receives a Melonite Treatment. It's a
salt bath nitriding process that imparts upon the barrel corrosion resistance and hardness similar to chrome, without adding a layer to the barrel.
I wish I could help you there. Ever since I purchased the 15-Sport, the urge to purchase a Mini-14 diminished.
Maybe a Mini-30...but then might as well just get an AK...
I haven't had any parts failures on mine. I haven't read about any chronic parts failures.
The 15-Sport is an AR platform, minus the forward assist and dust cover. You can use any standard AR part on the market on the 15-Sport.
Additionally, you can easily utilize any AR aftermarket accessory (furniture) that is made for a carbine length, direct impingement AR, with an A2 front sight, and mil-spec collapsible stock.
The great thing about the 15-Sport is that it is an AR platform. If there are any mil-spec parts that you absolutely must have, you can install them.
One 15-Sport owner here (apologies for not remembering who) replaced the 15-Sport's upper receiver with a mil-spec upper receiver that has a dust cover & forward assist.
A few of us are experimenting with different buffer weights & spring combinations.
Some have installed after-market trigger systems. Some (like me) tuned the stock combat trigger.
This is the truly addictive part of owning an AR platform rifle. It's the adult version of Legos.
Yes it does.
Depending on the firearms restrictions in your area allow it, the 15-Sport normally comes with a 30 round Magpul PMAG. This is considered to be one of the best, trouble free mags on the market.
I'll touch on one more subject that is important to me. Which one is less complicated to field strip & reassemble?
Ruger Mini-14:
Ruger Mini-14 Disassembly - YouTube
AR-15:
AR-15 Field Strip - YouTube
Now see an 11 year old girl field strip & reassemble an AR-15 in 53 seconds.
McKenzie -11yo Girl Sets New Record for Field Stripping AR15 - YouTube