Well lets clear one thing,f14 was selected because it outperformed f15 in every aspect,f15 was technologically far behind in that time.It was not random selection ,Iranians sow both presentation and they explore both solutions and f14 was logical choice considering Iran situation.
F15 could fire aim7 and aim9 and,both sidewinder and sparrow had better performance on f14(range and lock on perfomance).Radar on f14 is best radar ever produced, even today there si no aircraft with such perfomance.So even without Aim-54 F14 was better.If we add aim54 than it is clear choice.In terms of bvr f15 Iran could buy had almost same capabilities as f4 since they had same BVR weapons... Better radar could increase BVR capability but not that much and it was still less than F14 even withoit aim54.
Again f14 was so advance that even today there is nothing close to F14 performance.
Data link...can track 24 targets and engage 6 in same time...for comparatiom J10 can track 6-10(depending on version) and engage 2 in same time.This feature is what separate big boys from point defence fighters...when you have massive attack from air...only few f14 can engage 2 squadrons... Thus if you hide 2-4 migs behind it is scary combination..Now I didn't even mention Aim54, missle that prove itself capable at 5km and 190km range...I'm not sure everyone understand what f14 mean,it provides Iran with strategic capability at same way s-400 provides boost for air defence.
Ofcourse such complex system is expensive and f15 has own advantages but not in terms of combat performance and expecually in role it is used in Iran and primary role in Iran is air defense,for that role it is better option 3 times.
We are talking about f15 with air to air wepons same as f-4 and without multirole capability(no aim120,no smart ammunition) ...but again even by todays standard you will not find anything that will outperforme f14 air to air capabilities and in air defense role it is what is most important.
Downsides would be...price...no multirole capability which mean you have to maintain more different types of aircrafts and that is by definition very expensive...