Differences Between Terrorized Acts

Aside from the choice of end boss, the main differences are in average number of elites (still relevant for heralds for now as well) and in the mob types you encounter.

Avg elites give you an order of 'best acts' as follows:
1) Act 2 with 231
2) Act 1 with 220
3) Act 5 with 183
4) Act 3 with 161
5) Act 4 with 51
(credit for those numbers goes to Pavke)

Realistically, acts 1 and 2 are near equal in that regard and take first place.
Acts 3 and 5 are then again close together but noticeably behind acts 1 and 2.
And we all knew act 4 is way too short to ever win anything.

With regards to mob type, the benefits of each act depend a bit on your class/build.
A2 for example has a fair few magic immunes in the tombs which isn't ideal for say a zerker. Especially if you're talking about T5 heralds.
A1 on the other hand tends to have mobs that are exceptionally easy to kill but

Fallen also keep running away after one dies which can be annoying and delaying in its own right.

Personally, I tend to switch it up between A1 and A2, depending on the build I'm on or really just depending on the mood I'm in. They feel a lot more efficient than the others when it comes to full act clears. Though A3, 4 and 5 do make for some nice selective rotational TZs. So ideally, timing permitting, just combine both: run a rotational TZ outside of act1/2 to get a couple heralds prepped (to make it worth it) and then shard a1/2 from there.

Though depending on what really ends up happening with the

Herald changes now that they've gotten some PTR feedback, that might all become a lot less relevant.

Act 1, for now at least, is also simply "cheaper" to shard as those shards drop way more often than the others. Though that seems to be changing next patch as well.