It should be cost-neutral (or only be a little bit more expensive), but the pricing is sometimes messed up. It is safer to get a quote for both and then have the subscription quote fixed to match the co-term quote.
I use subscription licencing a lot (almost exclusively) with larger customers.
The first problem is with kit going EOS.
Lets say you want to buy a 5 year licence to get the maximum savings. But you have switches (or MXs or ...) going EOS in 3 years.
With subscription licencing you can replace that kit whenever you like (such as in three years time). You don't have to replace it at the same time as you buy your licences.
This also helps spread out the spend. Rather than one giant lump, you can spread it out as devices go EOS over the 5 year term.
The second problem is the time to roll out new kit. Lets say you are using co-term licencing, and at the same time you are replacing 200 devices spread out geographically. Once you apply your new co-term licence you have 30 days to complete the hardware swap out. After that you have a painfull licence grace extension to process every 3 weeks or so until you complete the process.
With subscription licence you can take as long as you want to replace the kit.