This is a very complicated question.
- Execs are very good against armour and high toughness (S6). TG are decent against armour (S4 AP) but bad against T6 (wounding on 6s).
- TG are very good against numerous enemies (2A each). Execs are not bad but being 1A, they might have trouble carving through big ratmen hordes and the like. S6 is often overkill in those cases.
- TG are better against extreme toughness (sphinx with T8) because they wound on 6s as everyone but have at least more attacks.
- Execs are very good in the first round of combat (hatred) but their damage goes down considerably after that (1A each). TG are more stable in this regard.
- TG are I6 while execs only I5. This is very important when fighting, say, swordmasters. I am lazy to math it out but I would assume that swordmasters kill executioners horribly while the fight against TG is roughly even.
- The best buffs for Execs are bonus attacks - light magic, altar. The best buffs for TG are bonus to strength - black magic or wild magic signature. Both are easily accessible but the magic support can be put right inside the unit.
- Execs cannot be combined well with Yema characters and altar of Yema. TG can be combined with anything.
So all in all, I think the choice is not clear.
It depends on what the horde is supposed to do and what will you leave to other units. If you go TG, you will need something to deal with high toughness. If you go execs, some anti-horde tools might be handy.
The current popularity of TG might be partly caused by them having been very weak in WHFB. Everyone knows how to play execs but TG are a new challenge.