I would personally go with the internal card. At the very minimum, there would be one less cable to deal with and one less set of connections. The only reason to go with the external box is if you are going to go to something that is really notably better than the basic Project outboard box.

Typically, if it is an add-on internal board, then the board likely has less of a profit scaling tied to it, as it is designed for the one application. It is not designed to pass through marketing channels like a fully blown separate sale item and it has no fancy packaging or external power supply considerations. These things add up to give you bang for the buck on the internal card vs outboard ones, re money spent vs quality achieved. others may feel differently, though, but I'm guessing due to the cash out vs what you are getting, and on the internal card, less money in boxes, marketing, etc,and then no expense for the power supply..and the lack of cables and connectors..well, this combines to give you very good quality. This is with both items being new, though, regarding cash spent.

For example, sometimes on audiogon there are the odd 'black cube' outboard boxes, used, in the $200 range. That would be a step up, IMO, and might be notably better than the Arcam Internal card or the basic Project box.

i own the diva a80. they probably use the same phono board. for moving magnet cartridges, it is excellent. very controlled, good extension. had i not gotten a package deal that included a lukachek phono stage, i would have used this with a step up when i made the move to moving coil. in fact, i am simply using the entire unit as a preamp with a cary tube amp i purchased and it works very well.
good luck.

tom