The smaller packs just cover a smaller area - they go down to the same level of detail.
I decided to go the same way as you - flush mounted 188C, CD ROM and datacard. The other advantage is that you can update the firmware easily everytime Garmin update it (they send you an e-mail, and you just download it to your card, plug it into the unit and way you go). I've been sent 4 or 5 updates in the 2 years I've had the unit.
I unlocked 1 area on my CD ROM - but as it covers from southern Scotland to Lands End (and a bit further) - that pretty much covers my needs!! I bought a reasonably big datacard (128Mb) and it gets all charts and plotting data I've ever needed or wanted on it - my basic 'load' covers all the charts from the Irish Sea from Isle of Man down to St David's Head. Remember to save the tidedata 'map' to the datacard as well - otherwise your tide prediction feature won't work.
Hope this answers the Q!!