When I got my Plug, NAND technology was new to me, and I had to read up on it. One of the reasons I quickly moved to an SDcard was that I wanted to experiment w/o any fear that something I might inadvertently do would ruin the NAND memory on my Plug. An SDcard, as you say, is replaceable, but the on-board NAND is not.
I don't hit my SDcard very hard - at least not yet. So far it has held up well. I know that rewriting inode access times is intensive, but for some things, like mail files, it is desirable to know whether they have been read since being written, i.e., whether you have new mail or not. That's why I settled on "relatime" rather than "noatime" - it alleviates most of the writes w/o actually breaking programs. (It would be nice if the ext fs's allowed you to set this feature on a per-file basis, wouldn't it?) And, BTW, I create all my SDcard file systems as ext2, not ext3. I suspect this probably has more of an impact on minimizing the number of writes than "relatime" vs. "noatime".
BTW, I'm not sure if specifying "relatime" or "noatime" on the Uboot parameter really works. I also specify it in my /etc/fstab. Between specifying it both places, I figure it will be honored.
The subject of wear leveling is a complex one. For a good primer, see: http://www.storagesearch.com/siliconsys-art1.html
Frankly, I don't understand how any SDcard can get by without a static wear leveling algorithm. With a jffs2 FS over raw NAND, the information about which blocks are free (and therefore available to take part in the wear leveling) is available, but on an SDcard, the NAND blocks are internally mapped to conventional 512-byte blocks and the card has no way of knowing which of these are free and which are not. Thus, the card must treat all blocks as potentially containing valuable data (right?), and either it must have a fairly large excess of physical NAND memory to serve as its free pool (which would be expensive), or it must use static wear leveling (again, right?). (If anyone can shed more light on this, please do so. Also, if anyone knows if there is some way to cause the card to read out the stats on its internal data on block write counts, etc, I'd love to hear about this.)
FWIW, I am using the following card: http://www.microcenter.com/single_product_results.phtml?product_id=0304661
I bought this card because a friend tried it and said it works well with the Plug. Empirically, it appears my Plug issues a write to the SDcard about once every 45 seconds on average, so statistically, the card should last a while. As it turns out, 16GB is wretched excess for me, but in today's world real-estate is cheap. And, so far, so good, although, I hesitate to add, my friend's card just recently failed. But, it's warranted for 3 years, so we'll see how it goes.