My cons may be pros for someone else, and vice versa. But I'll give it a shot. For reference, I supported Paul but am now going to vote for Kasich in the primary because I think he's the least objectionable candidate and has experience.
Rubio:
Pros -
Seems to be willing to reach across the aisle
Talks substantively about issues like education
Probably the most electable, and the most able to unite the party
Relatable life story
Knowledgeable on foreign policy (but he is hawkish, which I personally see as a con)
Generally very polished
Has an optimistic vision
Cons -
Inexperience - for reference, his career track is roughly similar to Obama's
Too "scripted" - see robot debate moment
Confusing history re: immigration (this is why Trump and Cruz supporters tend to not like him)
Foreign policy hawk
He's now the establishment's chosen one, with all the financial backing that entails
Cruz:
Pros -
Probably the most principled conservative in the race overall
Most willing to cut government spending
Very sharp and intelligent - probably the smartest person in the race
Good command of the issues
Dissatisfied with the status quo
Cons -
Doesn't come across as genuine
Not well-liked among his colleagues (not a single endorsement from a sitting senator)
Flip-flops on certain issues
Throws red meat to the base constantly (for example, he said he was going to "carpet bomb" ISIS)
Talks and acts like a televangelist (some people like it, but I think it's creepy and unbecoming of a president)
Very dogmatic
He's running a very slimy campaign
I would vote for Rubio before Cruz almost any day of the week if forced to choose between the two. I agree more with Cruz's politics but think he's an objectionable human being.