Now, I'll say right away my preference is to field Destructors. The main reason is primarily due to its cost (its 110 points). Another is duality. While some people dislike that term, it really is an important one in 5th Edition. Duality means that a unit may fulfill more than one role on the battlefield with at least average success. Destructors have a normal role of anti-infantry fire. However, you can easily conscript them to become light-armored vehicle hunters. Raiders (Dark Eldar), Orc Trukks, Rhinos, Land Speeders, etc all come to mind here. 6 Str 5 and 2 Str 7 shots a turn at a AV10-11 is scary. They also do exceedingly well against Monstrous Creatures with their cheap torrent of fire armaments.
The Annihilator is essentially the king of tank hunting capabilities for the space marines IF its allowed to fire. The problem is, its super expensive at 145 points and provides very little in the way of duality for a BT player. Sure, it'll likely penetrate whatever it shoots at. But what happens if you have no mech to shoot at or have run out of targets? You have a very limited use vehicle thats forced to shoot 3 lascannon shots at infantry, which likely will have 4+ cover saves. Good game. It is decent when firing at Monstrous Creatures, but you're not going to scare any of them. Most have 4+ wounds and can't be instant killed because of high toughness. PLUS, those MCs will also have those cover saves. Why not just try and put 8 wounds on a MC rather than 3? Doesn't work nearly as well in my opinion.
If you need AT fire, I'm a big advocate of the Destructor mix. That is the Autocannon/2 Lascannon kit out. This clocks in at 125 points, which is 20 points cheaper than an annihilator with 3 lascannons. What this does is gives you one more shot against MCs, infantry, lightly armored vehicles, AND you retain your ability to punch into heavily armored vehicles for 20 points less! Another advantage that is often not discussed is that many opponents may often discount an AC/Las mix because they don't think it'll present as much a problem to them as say Land Speeders might. This may allow it more turns to fire, whereas, your opponent will immediately target your Annihilator (hereby Trilas) as its a blatant AT unit that he fears. Would you not do the same? He needs his mech alive as much as you need yours, so Trilas often find themselves on the receiving end of AT fire. And the Predator's AV 11 side armor doesn't help the Annihilator's cause. At least the Destructor is cheap! This isn't even hypothetical situations or tactical planning on my part, its formed from dozens and dozens of games of experience using our current codex.
It is an innate trait for people to react defensively against things that "scare" them. Trilas is very scary to mechanized lists and will die early (or be chain stunned). A AC/Las mix might get away with a turn or two, depending on your opponent's target priorities. Even if it didn't, I must reiterate lack of duality point. Trilas just don't function well in roles other than AT. Destructors can, Destructor mixes can (albeit only slightly better than a Trilas, but is 20 points less!).
"In the end, it doesn't even matter." You must tailor your lists to your area's metagame. If all you play is Leafblower IG lists, then perhaps a few Trilas is a good idea. But don't expect to be competitive in a non-comp tournament (i.e. 'Ard Boys/Grand Tournaments), as you'll be facing a much more diverse group of people. Ultimately, do as you wish. But just keep in mind what was said above. :(