Obviously I don't know anything about your flying skills or experience level. But, as a CFI you'll have to teach in both those airplanes, so in theory it shouldn't matter if you fly the Archer for a little now or not. You should already be proficient in the manuevers, having done them for your Commercial ride and more recently in practice for your CFI ride, so the focus is on how well you can explain them while you're flying, pointing out errors, and other CFI "stuff". An Arrow and an Archer aren't "that" different and it behooves you to be able to be perform up to standards in either of them.
I had a similar kind of situation. I had done most of my CFI training in a 172RG. Checkride time comes, and during my oral, my instructor pokes her head in and informs us that the RG just went down for unexpected maintenance (hydraulic leak in the gear system). We finished the oral, the plane still wasn't up yet, so the inspector and I explored some options. I decided to do as much of the checkride as we could in a 172N that was available. Now, I hadn't flown an N-model much in about 20 years, but it was either that or just postpone the whole ride until the RG was back up. I figured that if I wasn't able to pass a checkride in one of the most ubiquitous trainers around, I definitely needed more work!
Well, we did everything that didn't require the RG, landed, he issued me a letter of discontinuance, and actually a few hours later the RG was back up, we came back to the airport, and we finished the checkride that day.
Moral of the story? Had I not been willing and ready to take the checkride in the N-model, I would've had to come back another day (and with winters in Ohio, who knows how long that would have been). I REALLY didn't want to have to come back later - all that self-induced stress and pressure for another day/week/month? No thanks!
Who knows? The day of your checkride might come and you could have the same exact situation as I did.
