They are both very decent guns, and so long as you pick a decent gun (and by decent, I mean a reputable make known for having good internal workings), then going off looks is as good a way to choose as any airsoft weapon.
As I'm sure you are aware, all airsoft guns (apart from one or two obscure ones), fire 6mm plastic BBs, so the only major difference between, say an AK47 airsoft assault rifle, and an MP5 airsoft submachine gun, is the internal mechanisms and their quality, as opposed to the real AK47 or real MP5, where they are hugely different in that they fire completely different ammo to each other, are built for very different roles, and are built to two very different standards or workmanship, the MP5 being built like a Rolls Royce, and the AK47 being built more like a jeep. But with airsoft, any MP5 or AK47 could be built either like a jeep or a Rolls Royce depending on which company is making the thing, so when it comes to choosing an airsoft gun which looks like a real world gun, your guide to quality is the company which makes the airsoft gun you have your eye on.
ICS is a company based in Taiwan (which is a good thing), and they are known for making very good weapons, with internal workings which are extremely durable, the motor's gearbox in an ICS airsoft weapon is modular, it is made of metal, all the gears are metal etc, so you are talking about robust, decent quality and good design when it comes to ICS airsoft guns. They do build cheaper ABS plastic guns, as well as more durable metal ones, but regardless of that, the quality of the internal mechanisms is invariably top notch. So there is very little you would need to do to either of those guns you linked to, they'll be great straight out of the box, although be sure to clean the barrel when it is new, because even new airsoft guns often have a bit of crap in the barrel (it will come with a cleaning rod, you just need to thread a tiny bit of cotton rag through the end of the rod to use it).
Having said that, of the two you are looking at, one of them is a standard real world weapon (the MP5 A5) which is much favoured by SWAT teams and the like, the other one is a customised version of an M4 of the kind which would be favoured by SWAT teams as well, but could easily also find itself on a battlefield. And although that M4 is not actually a standard real world M4 that one could buy off the shelf, it is one that could be created in the real world if we bought an M4 and customised it with a lot of after-market bits and pieces, because the M4 is the most popular airsoft gun by a very long way, and so like its real-world counterpart, there are loads of after-market bits for airsoft M4s, which means that an airsoft M4 is an excellent choice of weapon which is very modular in terms of you being able to modify it, which is why when most people ask 'what airsoft weapon should I get to start off with?', almost everyone always says, 'get an M4', although in fairness, most people do in fact usually say get a G&G M4, since they are inexpensive and reliable.
But beyond that reason alone to favour the M4 over the MP5, is that particular M4 which you picked, is as it comes, much more able to have stuff attached to it, to make it preferable for CQB and longer ranged fighting. Notably the fact that it has a lot of rails on it, which would allow you to add optics, lasers, tactical lights, grenade launchers etc, etc (yeah I know the MP5 already has a light, but it could get shot out, which could happen to an M4 too, but it's a lot easier to get a replacement light to fit on a rail than it is to get one that fits in that slot on the MP5). The M4 also has a much more robust collapsible stock than the MP5, so would also be slightly better suited to longer ranged fighting. Another reason to favour the M4 is that it has a larger magazine capacity than the MP5, which means you are less likely to run out of ammo at an awkward moment when at your skirmish, and additional mags for it would probably be cheaper to get hold of as well.
I'm not saying the MP5 is a bad choice overall, I have two of them myself and they are lighter and generally a bit better in a tight space, but I've also got two M4s and a lot of other guns too (lol, trust me, if you are anything like the rest of us, you'll end up with loads of weapons as well!), and I almost always end up using the M4s, but if you like the MP5, go for it, because it will still be a good choice.
. 
