The best thing you can do for advancement is to rank up your starter champ to 6 stars and level 60. I always recommend people make that their first goal. The difference in base stats between a level 30 champ and the same champ at 60 is astronomical.

The difference in base stats between lvl 60 and lower levels is gigantic, like harleQuinn said.

You can see the stats a lvl 60 champ, fully ascended will have in the index. A starter champ for example, as I have a lvl 40, rank 4, ascended to 4 stars Elhain on my account to compare:

Lvl 60: 14,370 hp, 1233 atk, 57% crit-damage.

Lvl 40: 7,590 hp, 619 atk, 50% crit-damage.

The difference of the base-stats is really important, as these base-stats are a factor for stat enhancing gear. Lvl 40 Elhain wearing a 60% atk armor: 619 x 1.6 = 990.4 atk. Lvl 60 Elhain wearing this armor: 1233 x 1.6 = 1,972.8 atk.

Thanks, but you didn't really answer my question.   The 3* abilities suck compared to getting an epic or legendary character.  It seems as those when you advance from 3 to 4* you do not gain a natural Epic toon is that correct?  If so you are better off sinking your resources into an Epic or Legendary instead of ranking up your starter toon or am I way off on this?

If the champ has any abilitie that can be upgraded by ascension, then this upgrade will happen at 3 ascension stars. This is true for all rarities. For example rare Diabolist (Demonspawn), A3, no ascension: Increase the turnmeter of all allies by 15%. 3 ascension stars: same effect + decrease the turnmeter of all enemies by 15%. Epic champ Aothar (Sacred Order), A2, no ascension: attacks 4 times at random, each hit places a 2,5% poison debuff. 3 ascension stars: each hit places a 5% poison debuff instead.

If a champion is worth upgrading is not determined by rarity. 99% of the players will prefer rares like Apothecary or Coldheart over epics like Yaga. The number of good rares is much smaller than the number of good epics, but there is a reason why it's often recommended to bring the starterchamp to lvl 60: All 4 of them are able to clear stage 12-3 brutal campaign on their own, alongside with 3 foodchamps. That (clearing stage 12-3 with only one champ in reasonable time) is the biggest milestone for account progression in the game.