Sticking with the question and putting aside whether it's a good idea or not, there are so many variables that go into the starting credit limit you receive.
- His personal and buisness credit scores.
- His personal debt-to-income ratio.
- How many new cards he's gotten (on the personal side as both cards are personally guaranteed) within the past year or two.
- Number of inquiries on his personal reports.
- His personal income.
- Business Revenue.
Generally speaking, for their respective business cards, Citi will probably give him the higher out of the gate limit, but he'll likely have to pay the card off frequently with either card with that amount of spend on it. Citi business cards do have bonus categories (bonus miles from those categories don't count for LPs). Barclays gives you a bonus of 5% of your entire base miles earned during previous card member year each time you renew the card.
The only real way to know is to find the card that best suits his business needs and apply for it and see what happens.