I am not very familiar with the cost of lipo batteries. The initial cost of lipo is lower than a123. However, lets look at the long term as one may.
https://skydrive.live.com/?cid=92088474ccb8cd05&id=92088474CCB8CD05!1730&authkey=!AJSyZwC_h7piGtY Look at the chart labeled 'cycle life'. After 3000 (three thousand) cycles of 100% DoD (depth of discharge) you will still have 90% of the batteries original capacity. Lifepo4 is often touted for it's excellent capacity and value. Most lifepo4 packs are claimed to reach around 80% capacity after 1000 cycles. I am guessing that after 6000 cycles, a123 will reach 80% capacity.
If you look at it from a value perspective, that is an intense value over the long term. A lipo pack is claimed to reach 90% capacity after 200-300 cycles. Likely 80% after 600 cycles. The way I see it, we can say the difference is cycle life is a factor of 10. If lipo costs 10 times less than a123, then there could be reason to go back and forth. I am pretty sure a123 packs may cost twice as much as a lipo pack. There are various things to consider like size and weight, but I've been shopping around for batteries for a while and a123 seems like a piercing and incredible value. Sadly, there aren't many sources of these batteries. Victpower, osn, em3ev
If you aren't doing any sort of racing or something, you'd likely never even come close to the packs C rating. C rating seems like a concern for many, and it isn't something to ignore with some lifepo4 in some cases, but a123 has a more than acceptable c rating when it comes with ebikes, even if you were to be doing some intense accelerating.
There are plenty of variables within this concept, but I feel confident in saying that most would agree, a123 will last at least 6 times longer than lipo. If you find the cost of an a123 pack to be less than 6 times more, than it's surely something worth considering. I'd also be confident to say that a123 packs are less than 6 times more expensive.
If space is something to consider, a rear rack may be an option.