I guess it depends on how often you use the boot and how much you plan to put in it? Tyre sealant doesn't always work so if you go down this route, make sure you've got roadside assistance cover. Might also be worth checking how many of the chain tyre centres stock your tyres. If the worst happens and the foam fails to repair the puncture, you'll need towing to a tyre centre and if they don't have the tyre in stock they'll need to order which could take 24 hours. Worth remembering that even if they do have the right size of tyres in stock, they may not have your preferred brand. If you desperately need the space, you could choose to mount the spare to the boot or go for a boot-mounted luggage rack.