If you have high HP and high Def, Heal Rune will be awesome for you.
Because it doesn't just "nullify 1 hit", such as you have 70000 HP, but opposing only deal like 2000 dmg on you, your Heal rune lv.1 heals for 7000, lv.2 heals for 8400. (that's 3~4 hits)
And if you're a Knight... place a Shield on yourself and if you have high Block, mass healing back to full HP.
Blood rune is mostly used to counter the above character, however lv.1~3 isn't that great until lv.5 and above.
3~4% of HP lost over 3~4 turns.
That's around 9~16% total HP lost (or around 1~2 hits from some characters - might as well just attack regularly since it takes around the same amount of round to fire off an attack or rune)
Lv.5 Blood Rune is 5% for 5 turns, and I believe it also can be cast 2 times. (so that's 25% of total HP - if used twice, that's 50% total)
Blood rune is mostly used in Class War by Mages and Knights, it is useless for Archer due to fast combat, no drags.
So for Archer, Heal rune is better if you have a moderately high HP and Def, best is if the opposing is targeting your Troops instead when you use a Heal rune.