Tough call. Personally I would do one of each, the carnosaur is just a fantastic model.
Maybe
@Killer Angel @Erta Wanderer @Kilvakar @ChapterAquila92 @Canas can give you better insight and some army list builds.
I'm not sure what I could add to the discussion, to be honest.
@Dr. Skinkers The biggest thing to keep in mind with the Seraphon: Start Collecting box is that you're not getting the proper unit increments for either Saurus Warriors (12 instead of 10) or Saurus Knights (8 instead of 5). Much of that is accounted for in the guide we have here in the forums for getting the most out of two of that starter box, but, barring any conversion work you may elect to do, you're going to be left with 5 models (4 Saurus Warriors, 1 Saurus Knight) that can't be taken as is if you do go that route. The same can also technically be said about the Skinks: Start Collecting box with the titular Skinks themselves (12 instead of 10), though that's purely down to the individual Skinks kit in general having 6 skinks per sprue (which, like those for Saurus Warriors, GW hasn't updated for a little under two decades).
As to where to expand from there, that's entirely up to you. At 1000pts, you're required to have a minimum of 1 LEADER unit and 2 BATTLELINE units, and may take up to 2 additional LEADER units, 2 BEHEMOTH units (including Heroes), and 2 ARTILLERY units. You may also allocate up to 2 reinforcement points among units that can be reinforced.
The biggest caveat is that no single unit in the army may take up more than 50% of the army's total allotment of points (personally, I wouldn't advise exceeding even 25% of the army's points on any single unit if I can help it - a tie-over from playing Horus Heresy).
With that said, just the two starter boxes alone will give you a neat and tidy 1000pt list with extras left over:
Saurus Scar-Vet on Carnosaur with Warspear (Leader, Behemoth) - 215pts
Skink Starpriest (Leader) - 130pts
10 Skinks with Javelins & Shields (Battleline) - 75pts
10 Saurus Warriors with Clubs (Battleline) - 105pts
5 Saurus Knights with Warspears (Battleline) - 110pts
Bastiladon with Solar Engine (Behemoth) - 250pts
3 Terradon Riders with Sunleech Bolas - 105pts
Total: 990pts.