Honestly, I believe everyone's opinion is just that...their opinion. I think the cheerleading for either side is pointless unless you have valid points to bring to the table. I have gained experience from others on which TPG they prefer and why, but most importantly I only determined which I liked from purchasing and examining them myself.
PSA scares me A LOT when it comes to pre war...just a good amount of inconsistency. PSA 3's and some higher with paper loss or just plain overgraded cards. ..but I buy any slab...just depends on the card of course
That being said, cards in lower grades can definitely vary within the same grade. I myself, prefer SGC, but do like PSA quite a bit for modern cards (high grade examples).
Golden Rule: Buy the card, not the holder!!!