The main reason that aromatase inhibitors (AIs) like arimidex, exemestane, and letrozole are avoided during PCT is because they excessively suppress estrogen levels. Realize that when you come off cycle, estrogen levels are already low (it's a derivative of testosterone which is low). If you take an AI, you're going suppress what little estrogen there is and drive it into the ground. That's not good for a number of reasons.
Estrogen plays beneficial roles in male health. Among other things, it has positive effects on blood lipids (cholesterol levels), bone density, glucose utilization, GH and IGF-1 production, and androgen receptor activation. Excessive reductions in estrogen have been known to produce lethargy and impair libido. The doctor and steroid expert Swale (John Crisler) argues that an AI "ABSOLUTELY should not be used post cycle (even though it has been shown to increase LH production) because the risk of driving estrogen too low, and therefore further damaging an already compromised Lipid Profile, is too great (this also drives libido back into the ground—and we don’t want that, do we?)." I agree with him.
In contrast to an AI, SERMs like nolva and clomid have selective effects, blocking estrogen where you don't want (such as the HPTA during PCT), while at the same time producing some of the beneficial effects of estrogen (e.g. blood lipids, bone density).
I'd avoid AIs during PCT and stick to a SERM. Nolva, clomid, and toremefine seem to be particularly effective for this purpose, in contrast to raloxifene. There's also evidence that nolva is superior to clomid at blocking estrogen in the pituitary. Also, take hCG during your cycle. That's the number one thing you can do to ensure a speedy recovery, as it treats the rate limiting step for recovery of natural testosterone production (i.e. testicular atrophy and dysfunction).
If you're concerned about safety of nolva and clomid, toremifene is great just by itself in pct. HG toremifene (fareston) is pricey but scientific studies have shown it's safety as well as how effective it is at stimilating the HPTA and blocking estrogen where you don't want it.