Options in semi auto:
1. AR
Advantages:
-Caliber change with just an upper swap
-Better stock sights (easier to adjust, 2 aperatures)
-Easier to fix if something does go wrong (don't have to send it to factory)
-Many options for customization
-Lower cost, readily available magazines
-Better ergonomics
-Easy to mount different types of optics with removable carry handle
-Carbines come with an adjustable stock
-Used in combat for the last 40 years
Disadvantages:
-No folding stock (I prefer collapsible)
-Direct Impingement (for those that have a problem with it, I don't)
-More "plastic feel"
-Magazines are viewed as flimsy compared to other designs
-Sightline over the bore is higher
2. Mini-14
Advantages:
-More conventional rifle feel (If this is something you want, I don't)
-Magazines are viewed as sturdier (I debate this because in the AR you can get HK magazines and SA80 mags, both are steel like the Ruger and both are less reliable than the standard STANAG aluminum mag, but it is the "internet view")
-Piston operation (if you need this)
-Folding stock available as option
-Ranch rifles come with scope rings to mount a conventional magnified optic
-Viewed as more durable on the internet (I have read the opposite from LEO armorers who have actually had to deal with a large number of them, but it is the internet view, search M4Carbine.net or Lightfighter.net)
-Often lower cost
Disadvantages:
-Not user serviceable without very specialized tools and Ruger does not sell replacement parts to civilians.
-Less ergonomic
-Viewed as less accurate by the internet, I have only experience with 1 of them, and for what it is worth, this was true. I don't know about newer ones.
-More expensive magazines
-Less available for customization
-No military use of any consequence (unless you count the A-team or Bermuda
)
-Sights are not as good, nor are they as easily adjustable
-Rock in magazines are MUCH harder to use from prone and low cover
3. AK in .223
Advantages:
-AK are believed to be more reliable
-Folding stock available
-Often lower cost
-Very overgassed, usually will overcome any variation in ammo and alot of dirt.
-Simple to take apart and field strip (except extractor)
-Quite a few accessories available for customizing
-Extensive military use in the 7.62 & 5.45 forms. Very little use in 5.56 (Poland Beryl maybe?)
-Stamped steel construction
Disadvantages:
-.223 AK's are not all the same (everyone uses a different magazine)
-Not viewed as being as reliable as the AK-47 in 7.62
-Difficult for user servicing. (Contrary to internet lore, AK's can't be built with a rock in a cave nor can you just replace parts like you can with an AR. You need tools and you need gauges to check headspace, not every AK part will fit every AK type. Chinese don't fit Bulgarian and neither will fit Russian. Try to replace the ejector....
)
-Poor sights. Particularly for older eyes
-Less ergonomic
-Rock in magazines without a bolt hold open (Even harder than the Mini)
-Harder to mount optics or other accessories
-Heavier than Mini and AR/SCAR
4. SCAR
Advantages:
-Everthing the AR has except accessories and parts
-Piston operation
-Folding stock
-Bolt/carrier/extractor almost identical to AR but better designed (25,000 round bolt life vs 8,000 - 10,000 AR)
-Some military use (only 800 or so were ever fielded), much military testing (several million rounds).
Disadvantages:
-If you think the AR feels plasticy, the SCAR IS plastic....
-More expensive parts, some skill needed for some changes.
-Very expensive
-Some problems with damaging certain optics
-Only STANAG magazines, no PMags or other Non-STANAG mags that will work in ARs.
-Even higher sightline than the AR
5. Bushmaster ACR
Advantages:
-Similar to SCAR
Disadvantages:
-Not as well respected as SCAR for reliability
-Apparently Bushmaster is hard to deal with on parts/accessories
There are other options: Daewoo, ACR, FN FNC, maybe some others. Most are not as readily available as these.
These are of course, my opinions and worth exactly what you paid for them.