Really now, when comparing stats, use something simple without basing things off of item gains and such for the ECL 17's.
Lets base both characters off of a straight 10's average for both, so they are equal.
Half dragon, 18, 10, 12, 12, 10, 12 (Total +7) Human 10, 10, 10, 10, 10, 10 (0)
Now, level 17 Half-Dragon and Level 20 Human
20, 10, 14, 12, 10, 12 -------------- 12, 10, 13, 10, 10, 10
WF+GWF+(+5 weapon)=+29 -------- same=+28
WS+GWS+(+5 weapon)+strength=+14 Damage-------same=+10
2h weapon=+16 ----------------- same=+10
HP by Average Half-D=144.5 ------------- Human=130
AC With +5 Full Plate Half-D=AC 27 ------------ Human=AC 23
Special attacks Half-D=Claw, Bite, Breath ------- Human=None
Attack Progression=29/24/19/14(weapon) and bite +24. Human=28/23/18/13
Skills=Same due to Half-D's int boost.
Feats=Human has 4 more at that level over-all.
Special Defenses=Natural Armor and Element Immunity, as well as sleep and paralysis immunity.
Special Qualities, Low-light visions Darkvision 60' (elf and dwarf combined anyone?)
Based on this information, I'd say the Half-Dragon might be SLIGHTLY underpowered, depending on what you make.
As was mentioned a Paladin would make an EXCELLENT candidate for Half-Dragons because of the Charisma boost (Bonus saves, healing, turning, etc.)
All in all, it depends on what you want your half-dragon to be, a fighter obviously isn't the BEST candidate. I'd say Paladin, Sorceror, or Wizard are best.
Calrin