Rainbow Six Siege:

Rainbow six has a wide variety of maps that are all destructible, this allows for the attacking team to chose their entry points at where-ever they may choose while the defenders need to reinforce walls and set traps for the attacking team to fall into, this allows freedom of any tactical or loud entry whitest allowing the defenders to block entrances, slow the attackers and make it easier to counter the attackers.
In Rainbow six you can kill both the enemy and your teammates (friendly fire), where you hit your enemy changes the amount of damage you do, for example; head-shots kill instantly, leg shots do less damage but put people into a down but not out state instead of killing them, arms do less damage but kill them instead of downing them and shooting their chest does normal damage. This means it very easy to die in Rainbow six.
Rainbow six siege also has a large cast of incredibly balanced, diverse and unique characters with special abilities that either consist of traps, blockades or ways of countering or breaching the opposing teams defenses. Characters in Rainbow six can be customized as well by unlocking outfits in the operators load-out. Selecting a character also denies your teammates from picking that character, this stops multiple people being the same character.
Rainbow six allows for endless replay-ability as you never know how a game will be played out and there is a constant supply of updates and new content. Also Rainbow six is stylized in a realistic tone, this shows the more serious setting of the game and allows for more realistic immersion.
Despite all of these great points, Rainbow six siege can be impenetrable to newcomers due to the large amount of experienced players and toxicity towards lower level players and trolls can ruin matches with little penalty, this can be caused by abusing the "vote kick player" function and abusing the ability to kill teammates, even though you can be suspended for thirty minutes, however doesn't increase in time very often.
Over all I believe that Rainbow six siege is an amazing game and I would recommend you buy it, but be aware of the downsides and annoyances withing the game and its community.
Over-watch
Over-watch is stylized in a more cartoon-like appearance to most other first person shooters, this gives a light hearted and joking atmosphere that can be compared with Team-fortress 2. However, this can be considered to be a complete opposite of Rainbow six siege's darker and more realistic atmosphere.
Over-watch currently has a total of twenty one unique and interesting characters with their diverse appearances origins and personalities that can all be shown in their dialog and emotes (actions/animations that are activated purposely by the player), despite the fact that they are grouped into four character types. Each of the characters are completely different, for example "Junk-rat" and "Bastion" are classed as defenders, however, "Bastion" has the ability to transform into a mini-gun and shoot the enemy team from a fixed position while "Junk-rat" arches explosives over short or long distances in order to damage or scare away (would be) attackers.
In Over-watch no characters are made for only one purpose which allows flexibility while playing by using he characters abilities and specials in multiple different ways, similarly to how the rainbow six siege operators can be used to further the teams progress or help shape situations in your favor and win the match.
Unfortunately, in Over-watch most maps are built in a way that only allows for one mode of play which means that there is notably less variety than other games such as rainbow six siege, despite this is made up for in the hilarious and unique dialog.
Over-Watch also has a loot-box system similar to rainbow six in which they give you multiple randomized items that can be sprays, emotes or skins. this also comes with a multitude of easy and quick ways of unlocking them, however over-watch, unlike rainbow six, also gives the option to pay for loot-boxes with real money.
Also unlike rainbow six, Over-watch allows players to create custom servers with their own sets of rules, this allows players to create games with their friends and create miniature game-modes that might not pre-exist.
Over all I believe that Over-watch is a great game with a large amount of options of play and customization despite its limited modes.