Scott,
Both are excellent oils for shorter oil change intervals (less than 10k miles). IMO the Delo is a little better.
One of the reasons is mentioned above, better cold weather pumpability than the Rotella. In Florida this is really a moot point for you.
The TBN of the Delo is higher 11. 3 compared to Rotella's 10. 2. In theory the Delo should hold up better and last longer in the crankcase. I'm not sure if this is the case in real life. Only oil analysis can answer this question for us, but I suspect it would hold true. I have seen good oil analysis from guys who ran the Delo 7k miles. I would imagine under the same conditions the Rotella would have come back with similar numbers.
Lubrication Engineers did some testing of oils and the Delo showed considerably less wear than the Rotella T. Their oxidation resistance, which is a number we can use to theorize how long an oil will last (useful when comparing oxidation resistance of other oils) were nearly identical. Give this the oils will hold up about the same during the uselful life of the oil in your crankcase. Again, whether this is true or not in real life can only be determined by oil analysis.
Walmart sells both the Rotella and the Delo (at least here in MI they do) so availablily may be a moot point for you as well.
I know Jessup recommended the Rotella to you, and I'm sure this is weighing heavy in your decision. There are a lot of guys on this board that are using these oils and I don't think you will find anyone with any negative comments on either. Utlimatley it's your truck and you need to run what you feel most comfortable with.