The 2 first bikes are road bikes, and the last 2 are cyclocross bikes. Don't just look at the prices. You should be going to bike shops where you can see the bikes and have the differences explained to you. Very few people ever regret buying a better bike, but many do regret that they didn't spend a bit more money at the outset. Upgrading to a better spec later on, even if it is possible always costs far more than the price difference. Look at this way: markup on individual parts at retail is about 3 times as much as markup on a complete bike. On top of that when you upgrade you buy parts twice, you bought those parts included in the original price of the bike and then again when you upgraded. Unless you know how to do installation yourself, you pay for that as well. Original parts are included in your original warranty, which includes errors in building the bike. Upgrade parts only have a warranty on the part itself, not the installation unless a dealer installs the part.