It's difficult for me to have an informed opinion on the 1x2 market for the Cefn-Airbus game, as I don't follow the second tier leagues. Needless to say, it is a massive game for both sides. Cefn have done really well so far in the Cup, winning at Aber (0-1) and Prestatyn (0-2) and in the round before that at Connah's Quay (1-2), which is also an impressive result as they are very likely to win the Cymru Alliance. But Airbus are no mugs, they are a better side than Aber certainly, and I'd say they are currently a harder side to face than Prestatyn as well. Prestatyn were 1.44 for their game vs Cefn and Aber were 1.63, out of interest.
It is interesting to note that second tier sides have generally fared well against WPL sides this year; in additon to Cefn's exploits, Rhyl came very close to beating Llanelli in an exciting encounter, after previously knocking out Newtown, Aber had to rely on penalites to beat Llandudno, ditto Bala versus Buckley Town, and Carmarthen were taken to extra time by Porth. This all suggests that there is not that big a gap between the two leagues, at least if you discount the full-timers and maybe Bangor. However I'd say you were in a better position than me to judge the difference in quality traeth, as you follow a side from the CA.
My call, and it is a speculative one and not for high stakes, is under 2.5 goals @ 2.4 (bwin), although I'm waiting for other bookies to price this up before I bet as there might be better prices on offer. Although the Welsh leagues are very high scoring, you'd expect the semis to be tight, cautious affairs, and I did a bit of research which proved this to be true; since the 2001-02 season a majority of the Cup semis have gone under 2.5 goals. I particularly fancy the Cefn game to go unders, but am tempted to just back it in both semis due to this stat. This will be a new experience for Cefn and Airbus, playing at a neutral venue with a cup final and potential European football ahead, and I fancy them to concentrate on not making mistakes defensively rather than pushing players forward. From what I've read Cefn are a long-ball side, who are known for their aggression, and organisation...a bit like a Cymru Alliance version of Stoke. They managed to keep out two WPL sides already, in Prestatyn and Aber, with both games going under 2.5. IMO the bookies are pricing this based on the league stats of both teams, and I think they're not taking into account the fact that it is a semi final. Would be interested to hear you're opinion on this traeth.