I'd side with Ionia.
Now, I know that the Freljord has strong fighters and intelligent people (such as Lissandra), and a lot more warriors than Ionia does ... However, I think that Ionia has a lot more mages than the Freljord. I know that magic doesn't necessarily mean victory, but given that Freljordians have no way to counter it (No demacian steel or anything right ?), the Ionian mages could launch a bunch of attacks even before the Freljordian could come to them. And let's not forget that Ionia has a lot of smart people too ( if Shen, Karma, and all the 'Balance is everything' people decide to fight, they can build up strategy). Last but not least, Ionia has Syndra. I know she's not all-mighty and all but ... Come on, she can effortlessly lift huge warships and half of a fortress. Imagine what she could do if she were to use all her strength ! And tbh, she could just send giants boulders or even some pieces of land, directly to the enemy. And, even if I'm not sure about that that one, her power keeps growing as time passes, so during the "1 year of preparation" she will get stronger. Okay I'll admit my judgement is a bit biased since Syndra is one of my favorite characters but still, she's undoubtedly one of the most powerful characters in lore (I mean, not taking into account Aurelion Sol, Bard, Kindred, etc), and she's not even at her full potential yet.
[sg-syndra]
Now, none of this can, or ever will, canonnically happen, but I'm just going with the "what if scenario".
Also, I apologize for any mistake in my spelling or else, as English is not my mother tongue.