Is it more effective to use masses of low tier units or should i focus on building high tier units?

thx

Both. The problem low tier swarms encounter is that they can very quickly turn into a Not Swarm. High tier are strong, but they can go down when swarmed.

Certain units within a given faction tend to be more valuable than others, so if you have to sacrifice and can choose what to sacrifice, then those less valuable units are the ones to sacrifice. In general, ranged attacks and good specials make a unit more valuable.

For example, Pikemen/Halberdiers are one of the least valuable Castle town units. Marksmen and Zealots shoot, Cavaliers and Angels are powerhouses, royal griffins are quite fast, and crusaders double strike. Your griffins are probably going to have trouble getting into high numbers because they tend to get to the enemy first and get ganged up on, while your swordsmen and pikemen may have some trouble getting into the fighting unless the enemy is quick.

Special abilities aside, it all comes down to math. The attack and defense of the commanding heroes will have a huge influence, and spells can upset entire battles. Generally it's not easy to produce so many low tier units to overwhelm higher tier units.

When I have flying units that are much faster than the other troops, I like to have them either wait or move to guard archers on the first turn. Sending them in alone usually means throwing them away.

In general mass low-teir units are most cost-effective than higher teir units but suffer from attrition. 100 HP kills ten centaurs but no dragons.

If you aim to zerg a castle and don't care how much power you have left after low teir units are often better. If you want an army that can deal with battle after battle without taking many losses higher teir units are where it's at.

Low teir units are greatly enhanced by 'Haste', 'Bless' and 'Prayer'. Haste because they're fragile, and letting them attack first makes them far more effective. Bless because they tend to have large damage ranges, and Prayer because it gives them a bit of everything. High teir units tend not to need as many buffs and tend to be better at resisting or mitigating debuffs. High teir units all almost all resist the devistating spell 'Berzerk', although the three 'might' type cities don't get that benefit. Berzerking low teir units usually means they kill each other dead.

Generally you should aim to get lvl 6 buildings ASAP, but after the Capitol. Even if you can't afford to buy the creatures yet, they will accumulate.
In the meantime an infantry unit's relative worth is tied to its movement speed. Zombies are virtually useless, since they rarely are in a position to hit anything, while Centaurs are a force to be reckoned with.

Also consider which units will benefit most from your spells. Some units benefit much more from Bless than others, for example.

They are equal, but with caveat. Take for example Stronghold units: if multiplied number per week by damage dealt they all will be in the same range. So it's not like useless peasants from Total War. Caveat is in unique abilities, speed, which also affects map travelling allowance and price-pefromance balance. First level units are cheaper for mass casualities and wasting retaliation attacks. But higher levels have better attack-defence rating, which helps in dealing bonus damage. I've found from mid game most units will be from different castles and leftovers for both opponents, so you will hire anything that resembles troopers and will be happy.