If you are looking for 80s sounds why are you limiting yourself to those 2 synths (one a rack rompler the other an FM synth that will need effects units and such to get sounding as 'nice'). Depends on what kinds of sounds you want.
If you want true 80s 'vibe' maybe you should be looking at some Roland Analogs? JX-3P, Junos etc. JX-3P can be had for less/same as a tatty DX7 and is analog but stable and has a fair variety of 80s sounds and nice Roland chorus/filter.
If you are looking at Romplers then JV1080 isn't really '80s sounding' it's more a workhorse sound module with 'generic' strings, pianos, synthy sounds etc (basically everything you could need pretty much starting out) but doesn't have specific 80s character. FM of course can sound very 80s but you may feel limited with JUST a DX7 (esp if you have no outboard/effects).
If you are only going to have one synth and you can only choose between those 2 then JV1080 is going to give far nicer results for a full 'mix', it has effects built in and all kinds of real/synthy sounds. DX7 can do Electric Pianos and very FM-esque tones and atmospheres but can sound quite dry without extra effects, though it IS the more 80s sounding of the 2 of course. It's hard to compare them though and say '80s sounds wanted' as there are other synths that will sound both NICE out of the box and be unmistakably 80s sounding (if you don't program them to sound different). I'd say get a Juno 2, nice little keyboard analog but again stable and mostly digital in the way it operates (controls etc) and sounds nice enough, you'll get plenty of PWM synth tones out of that, and some pads/strings and it will sound quite 80s. JX3P has more character and an even heavier 80s tone but the keyboard doesn't have velocity etc (DX7 does). so hmmm yeah... you may need to redefine your uses of the synth for someone to be more specific in recommending it.
Also you should say what type of 80s sound you are after, it can mean many different things from early analog 'electro rock' stuff (think Breakdancing
), British rock/pop (arps and creamy leads, Think Duran Duran), loungey/mainstream pop (DX7 E-Pianos think 'moonlighting theme'), Weird industrial/electro bashes and whoops (FM, Emulator, Synclavier think Depeche Mode), Overtly digital epic sounds sprinkled with pure pop (think D-50), late 80s 'house' 'music' (M1.. if you must
) There are so many different facets to the so called '80s sound' that no one synth can do it all, though some synths sound (with presets or through architecture) 'very 80s'. To be clearer about what would suit you you should say what sort of 80s sounds you refer to - also bear in mind a lot of 80s songs still used synths from the 70s (especially Moog and Older Rolands). Not sure where JV1080 fits in with that, but as I said if you just want a box of sounds to get you up and running and are going to be limited to just one synth then 1080 would be more useful than DX7 and you could always program it a bit or get some custom sounds to make it sound more '80s'.