With the caveat that I do not have ifrit (yet), just sig - I think that both are amazing units and worth getting both.
Sigmarus is, IMO, among the best nat 5*s in the game easily. The LS is huge, really makes stuff like giants b10 and HoH 2/3 runs much more stable, and he has fantastic utility (aoe 100% attack debuff! chain freeze/stun for days!) to go with his obscene damage. GREAT at bursting down crystals on the way to giants boss, great at bursting down everything really. Doesn't hurt that he's also a great bus driver, clears faimon hard VERY easily/reliably with just normal runes, hell if you use special farming runes but I wouldn't bother with that. He definitely requires good runes to be all THAT great, but will be fantastic pretty much everywhere once you do. On the note of runes, I personally like to run tanky/acc/speed substats rather than offensive substats on him - the extra tankiness, speed, and utility really help him stay alive to do his thing, while running rageblade, atk%/critdamage/atk% still lets him do ridiculous damage. Skill levels are great but not super required to be useful, you can function alright with skill levels of 1 across the board, but maxing them will amp up his utility to an insane degree.
Ifrit is (again, imo) VASTLY underrated - yeah his passive doesn't break stun/freeze but that isn't what gets you killed in giants/dragons b10 in my experience, it's stuff like def break (giants) and DoTs (dragons) - and a speedy Ifrit basically permanently negates all of them on a level that only really Velajuel comes close to. And uh, let's be real here, most of us don't have Velajuel. Add to that that he has fairly spammable AOE stun and DoTs himself, and he's a great anti-boss unit who's also good for a safe-ish, but slow, run on the way to boss. His LS also picks up where Sig's leaves off, being ok in arena but especially useful in ToA. The big difference with Veromos is that while he's not quite as rune dependent as Sig, he is MUCH more devilmon dependent - his base stun/dot chances are terrible, but go up tremendously with skill levels, and the cd reduction on skill2 is a big deal as well. The main difference is that Sig is usable at base skills and great with them, whereas Ifrit is... underwhelming at best at base skills and great with them, in my opinion.
Personally I feel like Sigmarus is probably the better of the two to prioritize if you haven't gotten good DPS monsters already, he's useful basically everywhere whereas Ifrit is a bit more limited in terms of farming and arena. That said, both are fantastic and both are worth getting for any f2p - I don't think there are really any units that actually honest to god -replace- either of them. Viable alternatives sure, I'm not gonna say Ifrit is outright better than Velajuel, but Ifrit's skillset is insane.
THAT ALL SAID, if you only have the mats for Ifrit rather than Phoenix, you might as well get working on ifrit I guess.