As far as the ship kits go it is cheaper, overall, to but a kit outright. However, if you cannot afford the price in one go there is the option of buying the magazine in weekly (whatever) installments. This is more expensive but does not require a large outlay at one time. There is another trouble though; the magazine seems to drag out the process as long as possible meaning you could well lose interest before the project is complete. Finally, it would appear that it is advisable to take out a subscription to the appropriate magazine to best ensure that you get the complete set, as they seem to disappear from the shops quite quickly after launch.
If you can afford it, buy the kit.