Can some one expand on the technical differences between this two options on the VPLS setup?

Differences are described in RFC4448 section 4.1, 4.2 and 4.4.1

Actually from RouterOS point of view there is no difference in operation for these modes (except specifying appropriate mode during PW establishment), because VLAN tagging/untagging actions described in RFC 4448 must be configured manually using bridges and VLANs. Tagged mode was added because there are Ciscos which support only "tagged" mode.