Well in short, if you want something to last 3-5 years, be prepared to take it for a service. One thing to take note is that a smart phone isn't really designed or even supported to last longer than 2 years. Lithium batteries only really last 500-1000 cycles which again is around 2 years if you charge once or twice a day. That counts as 1 cycle is = 100% of battery use, so 70% and charging , then going down to 30% and charging is 1 cycle (30 + 70 = 100); subject to other variables such as temperature and humidity. Samsung make solid phones, I use to go for Nexus', Pixels, and cheap Chinese flagships until I realised they just aren't using quality components. Take for example the Pixels Microphone, it's been terrible since the Nexus and it still hasn't improved, and the screen isn't as good as Samsungs yet Google are flogging them at £700+.
Another fact of the matter is that Samsung directly do research and development plus the manufacturing of their screens and batteries so it wouldn't be a bad bet they pick the best for their flagships especially after the Note 8 issue they had. Obviously you could get unlucky and the phone stops working properly a day after the warranty expires (there's always Samsung service centres).
To finally answer your question; it's always worth getting the latest smart phone as that will give you software support for longer plus any additional revisions they make to the hardware. Samsung sell dual sim S9s btw. You have to specify when you buy it.