For a compare-contrast essay, would you choose to compare Love/Hatred, Dickens's views toward the aristocracy/lower class, or Paris/London, and...

While the writer should always choose a topic that he/she is comfortable discussing and supporting, keep in mind that the title is A Tale of Two Cities and, as such, there are many parallels and comparisons drawn between these two cities.  For instance, in the famous exposition, Dickens writes in the second paragraph,

There were a king with a large jaw and a queen with a plain face, on the throne of England; there were a king with a large jaw and a queen with a fair face, on the throne of France.  In both countries it was clearer than crystal to the lords of the States perserves of loaves and fishes, that things in gereral were settled for ever.

Clearly, Dickens intends to demonstrate in his narrative the parallels of these two countries.  For one thing, there are several characters who have dual identities--one in England and one in France:  Charles Darnay, who is tried for treason in both countries, as well; Solomon/John Basard/Roger Cly; Dr. Manette; Sydney Carton, at the novel's end.

Comparisons can easily be made between the lawlessness in England that Dickens mentions in his exposition and the bloodthirst in France.  (Even Stryver is like the bonnets rouges [the "Jacques" as Dickens calls the revolutionaries] as he is for killing M. Evremonde when he hears the gossip at the bank.)

If you consider the dual roles and identities of several characters when they are in London and when they are in Paris, you will have no trouble drawing parallels and comparisons.  If you stay with your original idea, examine how the love/hatred parallels the theme of order/chaos. (see the link on themes for other ideas, as well.)

For some practical assistance, check out the site below on how to write a comparison/contrast essay. Good Luck/Bonne Chance!