I have got both. Top box IS better as it should be completely waterproof and secure, but it is also expensive, the box can be had cheap, mine was £20 off fleabay, but the rack for your bike will cost a lot, it cost £90 for a renntec rack for the top box to bolt on. Having said that, tailpack is much cheaper alternative, can carry pretty much as much stuff and with a rain cover will be waterproof, also has straps and stuff to tie extra things onto it. Both are good, if you can afford a top box and rack, then get one, if not a tailpack is a good buy, used on ebay is pretty handy, I had a set of tailpack, panniers and tankbag for £50 all in! Tank bag is also a very handy tool! I use one with my top box.