i wouldnt touch any iphone/ipad. after 2 years, apple would send an update to intentionally slow it slow in order to tempt you to update to the latest model. happened to my ipod touch, dad's ipad, brother's ipad2, sister's iphone 4s.
if your Nexus is running slow, try factory reset first. if that doesnt help, try another ROM.
if u do decide to get another tablet, might as well get 1 with LTE built in. it's always nice to have options.
They don't intentionally slow down products any more than Google or Microsoft do. Updates can do that, especially with older hardware. And sometimes the first generation of a new product (like the Nexus 7) has issues with things like flash memory management.
I just got an iPad Mini 2 weeks ago and it's very fast and fluid, and an all around good tablet. I don't particularly like iOS for various reasons, but it's really not bad on this form factor, especially for using it on the go for media consumption or playing games. Battery life is excellent: I get 2 days of moderate to heavy use without charging. I got the iPad Mini 2 (aka iPad Mini with Retina) for $300, a nice Poetic brand slimline case/stand for $8 on Amazon, and a cheapo Rocketfish stylus (not very precise since the iPad doesn't have an actual pen digitizer or palm rejection, but good enough for app interaction and sketches) for $1.99 from Best Buy. I'm a happy camper.

I use my Lumia Icon as a hotspot whenever I don't have WiFi and it works flawlessly. I would hate to pay for the extra cost for LTE in a tablet and possible plan change it would require.
A standing concern of mine of how iTunes behaves in Windows 7; is it as aggressive as I earlier described?
I can tolerate having iTunes on one of my (three) PC's if it isn't too much of a nuisance.
I only used iTunes so far to update my payment method for the app store, which apparently was required since the last payment method I set (PayPal, originally set when I had iPad 1 a few years ago) isn't allowed as the default for some reason in the app store. Other than that, I try to avoid using it, and will probably uninstall it soon. iTunes has always been crap, especially on Windows. I haven't noticed it attempting to make any changes other than trying to update (it might have asked to manage my media, but I usually decline any non-essential installation features).
I'm also considering a Windows tablet, so on that topic, are there any 8"-9" Windows 8 tablets anyone would recommend?
The Dell Venue 8 Pro and the ASUS VivoTab Note 8 are both excellent 8" Windows tablets with pens. VivoTab Note 8 also has GPS, one of the few that does for under $300. If you don't need desktop apps or the pen, though, the current gen Nexus 7 or 9 and the iPad Mini 2 are better choices for speed, and are likely to be updated for a good while. Only Surface-level Windows 8/8.1 tablets truly feel as fast as the rest, especially if you multitask in the least.