**DISCLAIMER**: This is a focus on the 2 rifles in a video game, not in real life.   This is with facts taken strictly from CS:GO.

The age old question for any CS:GO player is what guns to use.  The choices are similar to the ones offered in the first CS games;  the low-cost pistols, the powerful close-quarter shotguns, the run-and-gun SMG's, and the almighty rifles.  Unlike 1.6 or source, you have the ability to switch out weapons for alternatives.  Most of the switches are pistol choices (CZ75-auto in place of the tec-9 or five-seven, and the p2000 or the usp-s), but one of these switches, the M4A4 or the M4A1-s,  are a primary rifle that will be used fairly often in competitive play.
Having these 2 weapons were a great move by valve. Having a silenced and unsilenced version of the M4's allowed people to create more ways to utilize the rifle and play how they want to play.  The M4A1-S has 20 bullets in each magazine in exchange for better accuracy and  less fall off damage.  The extra $100 pays for the silencer, which makes it very quiet and untraceable through smoke. The silencer makes the gun a bit longer though, making it harder to hide behind off corners.  The M4A4 has a magazine size 1.3x larger than its competitor, and more than double in its reserve.   This allows you to spray down multiple enemies at once and pre-fire common corners.  The M4A4's spray is more resembling the 1.6 and source M4 and the GO's AK.  People could also enjoy the M4A4's cheep, cool looking skins without throwing an entire paycheck at the game. 

On paper the M4A1-S seems miles ahead of its competitor.  All of these benefits for what cost, $100 and sacrificing some ammunition? The silencer extension seems to be not a problem considering that you should never hold a super tight corner with a rifle. The 20 round magazine shouldn't be a problem either.  One of the only times you will see all 5 terrorist at once is if they are rushing a site.  On a full eco team, you can take all of them out with just body shots with the M4A1S-S.  Even if they have armor, they will have a combined health of 40 after you finish your magazine.  For new CS players, this gun has forgiving recoil, and view punch. These 2 features allow the newer player to "spray and pray", like in most first person shooter games.  It also lets novice players calm down and focus while shooting, a thing that view punch messes up sometimes.  Even if you run out of ammo while using the M4A1-S, you can buy a secondary pistol , like the five-seven or p250, to hold off people in a pinch. 

So why would people use the M4A4? The M4A1-S seems so much better than it, but the M4A4 has 2 things the M4A1-s doesn't have, Spray and skins.  Even with the recent addition of the hyper beast in the chroma 2 crate, the M4A4 has 2 more skins than the M4A1-S.  It's not just the amount of skins that people like about the M4A4, its the quality of the skins for what they are paying.  They both have 2 skins that are over $150 to obtain (real life money mind you), and they both have a rare drop that are incredibly expensive (A1's knight at $369 and A4's Modern hunter at $260).  The rest of the "basic" skins for the M4A4 are good looking and more vibrant than the ones of the M4A1-S.  Look at the M4A4's Dragon king, Griffin, and X-ray, all of them you can get for under $10 dollars (a factory new griffin is $2) and they all have very vibrant and complex patterns, or have a really cool design in the middle.  All of the M4A1-S's skins are all pretty bland, having a theme of colors thrown onto the gun almost like the paint application in Microsoft paint. A factory new atomic alloy is $15 and looks like someone soaked it in orange and black paint.  The spray is something that keeps people coming back though.  You can miss 1/2 your shots and still pick up that kill on that player you are fighting. The spray resembles the 1.6 spray for everything and the AK spray in CS:GO.  Sometimes it's hard to switch from a long, wide spray of the AK to the smaller one of the M4A1-S.  The M4A4 is a rifle that has almost the exact spray pattern of the AK, with the only difference of the AK hooking back after 25 is bullets.  The spray welcomes experienced players of 1.6 and source with open arms, and for some that is enough to use it over the M4A1-S. 

Now what do you use? There is really no answer.  New players that are not used to this insane view punch and the skill needed to aim may be more comforted by the M4A1-S.  While the more experienced players that are asking themselves this question may be better off with the M4A4, because then they only have to practice 1 spray pattern.  Even if you don't like either of these guns, you can roll with the AWP or pick up an AK from a dead guys body right?