Hi Nedim
Mega voices are not like RX voices; in fact only one other manufacture outside Yamaha can play Mega voice styles accurately without conversion, and to achieve this they had to program and design a whole new sound bank for use with Yamaha styles. (This has the added advantage that owners can add them to their existing styles)
As to whether they sound better or worse, is down to whether you like how the programmer has voiced them, and whether you prefer the Yamaha or Wersi (Hypersonic) sound engine.

Quick question
I have read all the Korg manuals on DNC and heard live demos, however as far as I was aware, the DNC only worked on Manual voices not styles, could you just confirm that they are actually available for the style. (The Guitar mode is a separate entity to DNC)

Here are 5 styles in 2 parts, (There are a number of seconds gap between the recordings) the first part is an un-tweaked EMC conversion to Wersi, and the second is played directly in the OAA, and again no tweaking was involved. (The name of the style is at the end of the link)
http://www.4shared.com/audio/HHk3zkSK/Afro_Cuban.html
http://www.4shared.com/audio/2S_7Qx94/Big_Band_Samba.html
http://www.4shared.com/audio/rJbUg45_/Country_Brothers.html
http://www.4shared.com/audio/LV2Tx1ju/Easy_Listening.html
http://www.4shared.com/audio/4vjoa6rE/Italian_Muzurka.html

Bill