The CYGS is more prestigious certainly, but not for the reasons you have stated IMO. There is hardly any off season to speak of these days, and the only reason we refer to it as an off season is because it's when ALL the players don't play.
However, there are periods throughout a season where a player would get more rest than he would in the 'off season'. Examples are if a player doesn't play between the Aussie Open and the start of Indian Wells, and also if a player doesn't play between Wimbledon and Canadian Masters.
I'm not aware how long the off season used to be back in Laver's day, but maybe there was a more obvious gap between the end of one season and the start of the next. These days that gap isn't there, and that's why I feel a non-CYGS and a CYGS are equal in this era. The only reason I say a CYGS is more prestigious is purely for nostalgic reasons.
first of all, theres an off season, albeit a short one.
secondly, the Grand Slam extends in a shorter time span. These days from January 17th (the start of the AO) until the 11th of Sept (the end of the USO) -i.e. less than 9 months which is considerably less than a 1 year and 2 weeks which is the time it takes to win 4 GS in a row. Clearly a shorter time span makes it more difficult as you have less time to prepare and to train.
thirdly, there's a bigger mental burden to win the Grand Slam as you're likely to face considerably more pressure to win the fourth slam of the season than you'd to win, say the first one to make it four in a row. Even if it is for a matter of more prestige attached to the Grand Slam, it is precisely that prestige which adds a special mental burden and a greater interest by the media and the fans. Needless to say, that mental burden of winning the last slam to complete the Grand Slam is greater than the mental burden to win 4 GS in a row.