This has been asked, hundreds of times.
The EX71 is far more comfortable (except for the sweat if you run with them - yuk!) while the E888 is comfortable only if you have the right sized ear (like the 268).
The EX71 provides much better isolation - keeping outside noise out to an extent which is dangerous if you're walking, running or cycling - and reducing leakage which might annoy your neighbour on the bus or tube. The E888 is a normal earbud.
The EX71 is easy to obtain in the UK, whereas the E888 needs to be ordered.
The EX71 provides very strong (but not very subtle) bass reproduction, thumping away like a good un. Those who like bass (to the extent that they need a big car for the sub-woofer in the boot, and who drive around shaking their windows and fillings out) tend to love the EX71, though in truth the new Philips beat them even for bass. But many think that the EX71 lack warmth and discrimination in mid and high tones (but what do you expect - their drivers are half the diameter) and sound a bit 'tinny'. Certainly every time I use mine (going into London and using public transport) I think they're great, until I come home and use the 888s again.
It's chalk and cheese basically. The EX71s are cheap earbuds which do bass quite well, but which are otherwise mediocre, while the E888s simply sound dazzling with clarity, fidelity and warmth. In sound quality, the consensus of opinion seems to be that the 888s are unbeatable at this price point.
As to comfort, it's entirely down to personal choice and ear size. If you find the 268 comfortable enough, you'll probably be ok with the 888 too.
But I wouldn't waste your money on 71s, personally, unless you need the isolation.