It depends on what you want, if you have found a used, or never used 2018 105 group and can save a lot of money then go with it, it's a very good and durable model that was called the 5800 series.
Really the differences are subtle, they did bring in 3 new cassette gear ranges this year, the last series only had two...big deal, you could always use Ultegra cassette if you wanted to. They also changed the front inner gear ring so cross chaining wouldn't be a big deal...so what? it wasn't a big deal before if you know how to ride a bike, and even the new system you shouldn't be cross chaining no matter what they say it can do! They made the hoods less bulky and designed it to be similar to the Ultegra...big deal again, I use the older hoods and have no problems, it's not the least bit bulky. They say the new derailleur is "light years" ahead of the old...BS! when it comes to mechanical derailleurs a quality derailleur from 20 years ago will work just as good or darn near as good, there is no light years ahead even going back 35 years or so ago when index shifting first came out, moving the shift lever from the down tube to the brake levers was not "light years" ahead because it's still the old indexing system! they just moved the levers.
So keep that stuff in mind when you read about the differences in this website I'm going to give you. Look, the job of these websites is to get you excited so you'll buy the newest thing, so they over glorify stuff by a lot to make you get all emotional and think with your ass and not your head. I've been cycling for over 40 years, I've seen all the marketing hype and I'm immune to it! LOL!! I was in the corporate world for 25 years too, I wasn't a golfer but I knew a lot of golfers, and they would come in to work and brag about some new wiz bang golf club, and I knew from cycling it was all marketing mumbo jumbo. So one day a corporate worker came in bragging about how he spent $2,000 for ONE golf club, and he bought the WHOLE SET! So that weekend he played several rounds of golf, that Monday I asked him how much better he scored in his games, he told me to shut up! He didn't score any better with his $2,000 a piece clubs vs his $150 dollar a piece clubs! And he never did get better with them till he went for lessons with a golf pro which is what he should have spent his money on in the first place and kept his old clubs. This mania to have the best stuff in cycling is just that, marketing mumbo jumbo to stir up buying mania, cycling world learned from the golfing world how to rip people off. Anyway I promised a website for you to read so here it is:
https://www.cyclingweekly.com/news/...105-r5800-what-are-the-key-differences-374671