You may have heard before the 40k is a completely different game at different point levels, I have no choice but to agree with this statement as I've been playing for a LONG time now.
You may say: "But you're just getting more options with the bigger points games"
While in theory this is true, in truth it is not. You still have all the same options, but you just need to plan better.
This is part of what I call the Metagame.
Some people will tell you that there is no Meta-Game, but i have to disagree with this. Meta-Game is not a reaction to what is going on around you, but a reaction to the twists and turns of the game itself.
Why is the game different?
Well, let's take armies of 1500 points:
You still need to fufill the FOC requirements so 2 troops and 1 HQ are required.
You fill those and you're looking at 1,000 points (roughly) of points to spend on the rest of your army.
2 Land Raiders? there goes half of the rest of your army. Now the termies to go in them, and you're left with all of 200 points to spend on everything else.
Not much right.
So, you drop the 2x termie/LR squads and start again.
This is what I mean by Metagame.
at 2,000 and 2,500 points you have a lot more leeway with what you can and can not put in your army and still make it cohesive.
So how do Tyranids work at 1500 points?
Well, first off to get the most bang for your buck, the Hive Tyrant goes away as an option.
Why? well, let's see....
170 gives you the base tyrant, Tac on another 120 for his extra wounds (2 Tyrant Guard) and make him more useful than just get shot to death as I run across the board for another 25 Points (Heavy Venom Cannon)
and you're up to 315 Points. Before you go to your 2 required troops choices. that is more than 1/5 of your army for your single required HQ choice.
This leaves you with only a few options for an HQ choice.
Tervigon (very useful as a troop, and also very useful as an HQ choice at 1500 point games. Take him with Catalyst for 175 points and you've got yourself a very resilient and potent HQ choice.)
Parasite of Mortrex (fun, but only useful in certain army builds)
Alpha Warrior (Nice Cheap choice, but if you're not wanting to run a Warrior Squad then he's out.)
So there you go. Take a Tervigon and you're good to go. less than 1/3 of the cost of the Hive Tyrant and at this point level, I think It's more powerful.
So I have an HQ.
Now on to Elites, Here it is a tossup between Hive Guard and Zoanthropes.
I chose zoans every time. Personal Choice, but you can take either. I'll take 3 squads of 2.
Need more Tervigons. I love monsterous creatures. So I add their 50 point tax and take 2.
4 Troops taken
Now, Personally I am very less than impressed by the Tyranid Heavy Support choices, don't get me wrong, there are some monsters there, but I'd rather take the Fast Attack MC's before the HS choices.
So I'll take 3 Harpies, with Heavy Venom cannons. (see I get better HVCs from the FA slot than the HQ slot)
This gives me an army resting at 1495 points.
For good measure I'll add 1 Termagaunt and fill out my 1500 point army.