Both of these games are a blast to play although Space Marine 2 is 60$ dollars and Boltgun is 22$ dollars so if money is a priority than either play Boltgun and buy Space Marine 2 later if you want or wait for a sale on PSN, or, Steam or, Xbox or whatever works really.
Space Marine 2 follows the story of one Formerly Captain now liutentant Demetrium Titus and his fellow brothers Gadriel and Chairon. He and his brothers face the threats of the Tyranids and the traitor leigon Thousands Sons. It's relatively an easy story to follow and is accurate to the lore even though some of the charecters did things I would disagree with. Although it does not need much attention to understand and to understand the first games story just watch a youtube guide summarizing it.
Even though the story is a bit loose at times it still does a great job at summarizing the universe as a whole. It takes you through battlefields of people fighting Tyranids Swarms and Heretic Marines and Demons but at the same time showing what happens to traitors who betray the Emperor. It shows inspiring speeches given by commanders to inspire their men to die for what they belive in. It has moments ranging from pushing up a battlefield to assist soldiers to valiant last stands where rousing words are given.
Although the gameplay, in my opinion really well done. Gunplay is smooth and with the different weapon options it really provides an experience that's really fun. Guns like boltguns and meltas feel really good and seem accurate to how they would be in the lore. Although the melee is a beast to have fun with. Due to you being a Space Marine you are able to take many hits with the caviat that you are fighting basic chaff and a little bit of speacilized units. Parrying and attacking feel relatively simple considering it's a third person hack and slack shooter. Although the online modes are a different story entirely but in a good way. There are multiple classes that each have unique abilities that are interesting to play in Pve and Pvp. With pvp everything is somewhat balanced because each class has abilities to get around other classes but at the same time could survive on their own. With pve everything is fun to play since it's just killing hordes of enemies but when you increase the difficulty you need to focus a little bit more but not that much.
To start of I have less opinions on Boltgun and Boltgun is considered a boomer shooter which is similar to games like Doom and Wolfentstein and specifically the 90s version. In Boltgun you play as Malum Caedo a Space Marine tasked with cleaning up the forge world Graia and if you have played Space Marine 1 or just know the story Capatin Titus was defending this planet from Ork and Chaos invasion and so when the ending of that game happened some Chaos forces moved in alitle bit after and Malum Caedo is there to clean it up. You are tasked with killing a Chaos Marine Sorcerer who is after a power source which acted as the main Mcguffin from the first game. You hack and slash at Chaos forces with your trusty arsenal. A main weapon you will be using is of course the Boltgun which in it's own right is a powerhouse of a weapon feeling not like a normal gun but as it's suppose to, as a rifle that fires miniature rockets that explode before impact. You have a Boltgun, Bolt shotgun, Plamsa rifle, Heavy Boltgun, Grav Canon, etc. Each weapon feels like they have a specific duty to fuffil like true a Boltgun could kill a Chaos Space Marine but it's more effective to use a Plasma Rifle. Overall I think the game is swell and for 22$ dollars it's an absolute steal.
To reiterate, both of these options are really fun and the positives outweigh the negatives really. If though you are not interested in these games, there are more options to play that are really interesting and I will include a link to Youtube and search up Bricky space Marine 2 and Boltgun videos.
Link to search up Bricky game videos