Hi Rosie. The main factor for ohms (and the resultant wattage) will be what type of ejuices you like. The 2.4Ω will be better with stronger flavors -
tobacco, coffee, cappuccino, whereas the 2.8Ω will be better for fruits and berries - lighter flavors. The 2.4 may 'washout' or even burn lighter flavors and the 2.8 might not be enough to bring out the best in
tobacco or coffee flavors. Some flavors can take both well but might taste a bit different - RY4, menthol, and some caramel/butterscotch can span the range of both.
One thing I've experienced with CE2's is that the XL's that were out when I was trying them had wicking problems because of the length, so I preferred the standard size even though it was less volume for
ejuice. That problem may be fixed with newer versions but I haven't checked into them lately.
And... the 900mAh eGo is going to have lesser volts underload than the true 3.7volts of the 18650 - something else to keep in mind. IOW, the 2.4Ω will run a bit hotter on the 18650 than the eGo. If you can afford it, you might get a few of each of the CE2's

But if not go with what ejuices you use.