Nemesis in my opinion has some of the best moments of post-Tolkkivarius, no contest. They introduce a lot of new elements that work very well, I think, while retaining that poppier side of their sound and just polishing the hell out of it. Unbreakable and Halcyon Days are good examples of them sticking to their sound while adding new elements. Fantasy and Abandon are straight to the point anthems. Castles In The Air has that lovely fusion influence especially in the solo, and the riff and chorus are killer. There are three oddball tracks that don't do as much for me - One Must Fall, Stand My Ground, and Out of the Fog - but they're still good. I think it's one of the darker Stratovarius albums, possibly the darkest.
Eternal on the other hand almost shimmers with happiness and positivity. My Eternal Dream is one of the best songs they've done since the late 90s and is better than most power metal bands' entire catalogs. Shine in the Dark, again, is a bit too Cain's Offering for me at times (I love CO, but mainly the super fast songs, not the ones like this) but I still like it. Lost Without A Trace almost sounds like it could've been an Elysium track.
Great bonus songs on both by the way. Fireborn, Giants, man this band just kills it every time.