Definitely hills: during the day the flat coast road from Flint to Prestatyn is HORRIBLE: lots of traffic and nothing much to look at. You will see it again (kinda) in the early hours on your return, but it should be quiet then and is a good through-the-night blitz if you can get a group working together, or hang onto a faster rider's wheel. Up and over Halkyn you get the moorland on Halkyn Mountain and lots of quiet lanes through livestock country with a fast descent to the charming St Asaph and the backroads to Bodelwyddan, which is a spectacle – I always thought it a folly, but it's actually a genuine olde worlde country house and the marble church is much older than it looks.
And the route over Sychnant is too nice to miss, especially with the quick drop down the other side: beautiful open hillside, peaceful, unbusy, and only 150m of climbing. You will see the coastal route on your return as the light's fading and all you get to see on the bottom route really is a cycle path alongside the A55 expressway, the old A5 around the cliff and the back end of a caravan park – boring!
If you don't do the hills then it won't feel like Wales: not because of the hills, but because of the quietness and the scenery. If you stick to the coast road then it feels like a Butlin's day out.
It's also worth promoting the "proper" return route across Anglesey: we returned via the A5 last year in a quick group, but the B5109 is a much nicer ride and only a few minutes longer.
