Valkyries
are for transporting in a Meltavet Squad and destroying a high priority target, or clearing a DZ with its MRP's and them dropping off a Squad to contest and/or hold an Objective that's further up the Board.
Don't expect it to last for long after disembarking said squad though, which is OK so long as said Squad achieves its mission.
Also, if it had fire points it would cost many more points than currently...too powerful otherwise.
Chimeras
are cheap and therefore spammable, and are best with a Multi-laser/HF and with 3 Melta/Plasma Vet Squads inside performing drive-by shootings..quite often these are suicide missions, especially the Meltas because of the close range required to get maximum effectiveness.
Speaking of Suicide..SWS's are ideal, but fragile seen as there's only 6 of them, but 3 SW's or Demolition Charges is brutal if they achieve their goal.
PCS's with 4 flamers/4 Meltas are great also...4 BS3 Meltas is just as effective as 3 BS4 meltas.
CCS's loaded with SW's is also great, and actually makes a better suicide Squad than Vets because they aren't scoring, so not as great a loss, well unless you rely on Orders a lot.
Chimeras also make decent 'firebase' type units with Multi-laser/HB/Stubber, and with Plasma/AC Squads inside.
Overall the real differences between the two are -speed and the tactical options you get with the Skimmer, namely the deployment options of Deep-strike, Scout and Outflank...(and the fun and awesomeness of the Valk/Vendi models).