Even with 4 days to play with the Nexus 4 before shipping off my sold Spirit it wasn't an easy decision. I came VERY close to rescinding the sale, keeping the Spirit and having Metro reinstate my $55 plan. (With the Speedtests I was sending them, I think that could have happened if I had whined enough.)
I think a lot of the decision should be how happy you are with the phone you have now. Taking signal strength into account, I had a better user experience with the Spirit than I'm having with the N4. The Spirit has been the best phone I've owned so far, By Far. That may change when I get around to rooting and enabling Lte on the N4.
Probably the main point for me ended up being the $130 I got for my Spirit. Unfortunately, after shipping, Paypal and Fleabay fees, I only netted about $102 for it. IF I had to settle for netting less than $100 for the Spirit, I probably would have kept it, and stayed on CDMA until about the end of July '14. The warranty for my N4 is up in August and I'd have wanted to have it activated before that to be sure there was no warrantied issues while I still had one. Doing that would also have effectively dropped the price I paid for my N4 another $30 (6 months x $5)
I also think that I was, and you still are, in the "sweet spot" for getting used N4's. The longer you wait now, the more use the phone gets before you get it. Right now you can still get units that are like new and still under warranty. The longer you wait, the less of those there are and the more they may cost. I paid $220 for mine, it was just 3 months old when I got it and still had the factory screen protector on it. It even came with the best N4 case, the Ringke Fusion ($15) which I would have bought anyway had it not come with the phone. It was also like new which means I ended up paying about $205 for the N4 itself. I DO have to admit the N4's wireless charging is the kitties jammies.
It also wouldn't surprise me to see Metro get some other new Mid-range GSM phone in the next 6 months worthy of consideration. OTOH, if tethering is important to you it Seems these days the safest option there is to go with a non-carrier branded phone.
The gist here is, at least for me, there were several angles to switching over that I needed to noodle out before deciding.
Bruce in Ocala, Fl