Thanatos - not doing something because it is hard...it like not eating something because it is healthy. It may be less enjoyable, but worthwhile in the long haul, so please don't think of terms of hard and easy - nothing in either book is out of your grasp with enough practice.
Expert at the Card Table is considered a more advance read, because - the more you know - the more you get out of it, as people have discovered much by reading between the lines and filling in their own gaps. It is a book that can be studied 1000 times, and you will learn something new each time - I even have the mini-bible like version that I carry with me at all time. If I was stranded on an island with a deck of cards and that book - I would be happy...maybe add in a super model for when I get tired of reading.
As for Expert Card Technique - that book kills also - Charlie Miller was one of the main contributors - along with Theo Annemann, Harold Lloyd, Jack Merlin, and Dai Vernon...just to name a few. It has just about everything you can need - and I have used it as a main resource for techniques and theory for years.
Card College volumes 1-5 (and even Card College Light) are also a good source - they are more updated, have attached theory concepts and packed with effects you may use. The books aren't really that expensive if you compare it to other great modern day works - you can get all volumes (and a deal if you get them all at the same time) for about $200.00 - if you take another great book like David Regal's new book - it is perhaps the thickness of 2 volumes - and it will run you almost $100. If you consider that if you master EVERYTHING in the volumes you would be one BAD A$$ magician with cards, then it is worth the investment on that alone. Again, it is more than just moves - you get tricks and theory for each thing - the amount of detail put into CC series is SICK.
Nevertheless, if I had to choose between the first two - it would be difficult - it really depends on you as a reader. Also, are you interested in gambling sleights - as half of Erdnase focused on table artifice. If you aren't - and you don't mind a bit of a dry read - I would recommend ECT - Hugard and Braue - as the technique that Charlie Miller used was so natural and disarming, with so much, that it would even fool those in the know.
Anyhow - as always - the choice is yours - but never choose the easy path, because that only makes you the majority - and do you want to be remembered like many others...or stand out when you entertain?