Any reason to cast Fortune when able to cast Summon Leprechaun

Generally, a summon is there to be a meat shield. You want it to take retalitations and attacks, while your real creatures stay safe and then strike themselves.

To do that, you want high speed, good defense melee fighters. They go in before your real troops, take the brunt of the attack and then your creatures mop up the rest.

What you want to avoid are casters, as illusions don't have any mana (that's a weakness compared to nature; summoned nightmares, water elementals or faerie dragons do have mana available)

In cases, where the enemy army consists of fairly slow melee creatures, you may want to clone a ranged creature to get more firepower.

But otherwise these creatures are your best bets:

• Griffin - fast, durable and unlimited retaliation
• Ghost - deceptively durable due to their defense, their low hp will summon you a lot of them
• Air elemental - see above
• Vampire - can sustain himself, also fairly low hp, so higher numbers. For vampires to work, you need to summon enough to be a threat, though.
• Minotaur - their block special is very good and can help them survive several attacks even if their number is low
• Efreet - extra damage due to the flame shield
• white tiger and nomad - very strong lvl 2 creatures

I'm not a fan of summoning lvl 4 creatures with illusions, that's another weakness compared to nature. You get 0.2 griffins per level, yes, but you also get 0.143 phoenixes, That amounts to 39 hp summoned per level and outclasses phantom image pretty much immediately. With hit point based summons, I think you are better off summoning larger numbers of weaker creatures.

Also keep in mind that summoning spells on heroes usually only make sense in campaigns. On a single map the hero will have trouble keeping up with army growth to summon an amount of troops significant enough to have an impact and espeically with order your are then better off using other control spells like blind or berserk.

Naga is in a bad spot. It's inferior to a genie in pretty much every way.

Even against magic immune enemies genies can just summon creatures to take hits, whereas you need to spend mana on nagas to get them into a fight and then you have a decent damage dealer.

That doesn't even account for strategies, where you split your genies into several stacks, using single genies to song of peace an opponent for several turns, while other creatures pick them off.

Nagas get the upper hand, once Genies run out of mana, but usually battles do not last long enough for that to make a difference. I always pick Genies over Naga 100% of the time.

But HoMM 4 is full of decisions like this. Cyclops vs ogre mage is another, where the ogre is so much weaker than the other choice, that you should never consider it. Griffin/Unicorn, Angel/Champion, Vampire/Venom spawn, Medusa/Minotaur, Gold Golem/Mage are other very lopsided matchups in favor of the first creature.