Well, we get those two letters "for free" anyway, and the phonemes are distinct in modern English, so let's use them!
"ð" also has the nice property of being sufficiently similar in looks to "d" that someone reading it and not familiar with the letter might pronounce it as "d" and thereby make a decent approximation. Unfortunately that's not the case with "þ" though. If we ignored the historical letters and instead just picked something reasonably sensible, I'd probably go with "đ" and "ŧ" for this reason.
i've had people mistake "ð" for an "o" with a diacritic, but presumably that's a typeface problem. whatever we choose, i insist that we drop the names and call them "eþþ" and "ðee", by analogy with our other fricatives ;)
"ð" also has the nice property of being sufficiently similar in looks to "d" that someone reading it and not familiar with the letter might pronounce it as "d" and thereby make a decent approximation. Unfortunately that's not the case with "þ" though. If we ignored the historical letters and instead just picked something reasonably sensible, I'd probably go with "đ" and "ŧ" for this reason.