Pathfinder is, as stated, the cleaned up 3.5.

However the sheer volume of material for 3.5 makes me stick with it, alongside the quality of the material.

I think I'm stuck in tradition- it was the first thing I came to learn about DnD and I'll probably stand by it unless 5e is amazingly brilliant.

I like both 3.5 and Pathfinder, although I'm definitely a bigger fan of old-school D&D editions over either of these two. However, they are in most every sense the same game, with some slight, almost unseen mechanical restructuring for the sake of cleaning up bugs left from 3.5 (which weren't many to begin with). D&D allowed Paizo to use the system because
1) WotC new many fans would hang on to 3.5, just as many still do to OD&D and especially AD&D 2e, and a new edition requires much manpower to keep releasing new material for the system as well as fixing things that turned out to be broken after it's release.

2) WotC wasn't (at least at that time) worried that Pathfinder would prove to be such a strong competitive product.

Of course, they've finally seen the error of their ways in that regard, which is the reason behind D&D Next (5e), which I've playtested with amazing satisfaction despite my initial concern

Really, there was only one big one. The spellcasting system.

I've never really had any issues with the spell-casting system in 3.5, to be honest

Mathematically and thematically speaking, it sucks. It's terrible for balance, and terrible for most adventuring tropes. And WotC cocked up most of the spells too, so most are useless and some are wtf broken/powerful (polymorph, for example).

But from your sig, and the fact you have stated that you are a big fan of 5e, I can already tell that your metric for liking or disliking things is vastly different than mine.

I don't disagree with you, I'm really just curious: could you elaborate what's thematically bad about it?

While bad may be too strong a word, it's certainly a little odd sometimes, mainly because it is different to what, I'm sure, many of us are used to.

When you play a videogame, for instance, you have mana points. With D&D, it's like you're using a revolver. You load bullets into the cylinder, and while you can "swap down" bullets (say, put a .38 special round in a .357 magnum), you can't "swap up" bullets (Say, put a .357 magnum round in a .38 caliber revolver).

The "recharge after 8 hours of rest and one day has passed" mechanic encourages the 15 minute adventuring day, in my opinion.

It also begs the question of "why aren't spellcasters providing services like ending hunger" and stuff like that.

Human selfishness already provides an answer to that question.

Because we (the players) still want kick-ass spellcasters inside a pseudo Dark Ages framework. So we actively ignore the consequences of having thousands of spellcasters running around.