which is better : XP COLLIDER or bonus % items

Your question is more complicated than you think.
Items that increase your Multiplier do so permanently.
The XP Collider has a temporary effect of raising your permanent Multiplier by 5. So if you have a permanent Multiplier of 6, then the XP Collider will multiply that up to 30. Please note that the X5 multiplier does not show up in your Conform-o-meter, just the permanent figure is shown.
You can purchase charges for the XP Collider in units of:

• 24 Hours
  
• 3 Days
  
• 7 Days
  
• 10 Days (best value)
  

Ok, now for some advice from a long term player (7 years experience and I'm a Hero in the EA Answers forum).
Hold off on spending your donuts until you've gotten all of the first 60 levels of content completed. After level 60 there are no "Level" characters or buildings to buy. They just start giving you donuts for each time you Level Up.
Now start buying Purple, Blue, Pink, White, and Orange houses. They pay for themselves until you have a big bunch, then they increase your income hugely. This is called House Farming. You'll need that cash bump...
Make a space big enough to hold 36 (or however many you feel is the right number for you) Kwik-E-Marts. Do NOT buy them yet, just make a space for them with nothing around that can be sold.
Save up 25 donuts. Easy peasy since you get a donut every time you level up. Once you get to level 939 it changes: you get a donut box for every million XP which can have 1, 2, OR 3 donuts in it (or you can buy 2nd and 3rd chances until you get 3 donuts).
Now charge the XP Collider for 10 days. During those 10 days you'll be buying Kwik-E-Marts, letting them 'build' for 4 hours, tapping them to collect the ENORMOUS amount of XP they produce, then selling them. Doing this will bump you up though the levels like lightning.
So now you're collecting lots of donuts. Start buying Mr. Burns Mystery Boxes from the Premium menu (Homer Budha icon in the store). Each one costs 6 donuts and it cycles through a pattern. This is the absolute cheapest way to increase your Multiplier. After a few months you can have a Multiplier like mine - the highest multiplier I have in my 5 games is over 2,400. I can produce over 100 donuts a day without doing anything else just by logging in every 4 hours and "KEM Farming". Here's a video of me collecting donuts in one of my games...

Oh, yeah... You have to have the Multiplier items out in your town to count, Can't put them in storage.

LoL file this under 'stuff I wish I'd done from the very beginning' :p

My multiplier 'sweet spot' is 2000%, given where I'm at in terms of space on my map and item count. I've got around 2000%'s worth in storage.

I don't send characters on tasks outside of events, so rent collection is the main path to level-up donuts. I also have the Lard Lad Donut Factory and CostMo which combined return 12 donuts every 24hrs.

Using Sky Finger once a day outside of events nets around 24 donuts a day. I found buying Blood Mobiles and Rattrap Trucks to level up a hassle. Whenever I need more donuts I level up by buying and selling The World's Largest Zirconia, which is worth around 18 donuts a pop.

Bonus % items for sure since they are more of a long term benefit.

Whereas any benefits you get from the collider are short term.

If you can't afford to run the XP collider all the time then just activate it when you donut farm.
Yes .25% four times will be 1%
The 5 Beach Hideaways, when fully upgraded, are the best value for bonus at 100% for 200 (I think) donuts.
A guide: https://forums.ea.com/en/the-simpsons-tapped-out/discussion/149536/kwik-e-mart-also-known-as-kem-farming-guide/p1

NOW I remember what I forgot to mention:
Permanent Multiplier items increase both the $ and XP payouts. The XP Collider only increases the XP.
And just as info so you know (and anyone else reading this) there is also a building called The Super Collider, and a character called The Collider.

I learned late in the game that the secret to large amounts of free donuts is to build a high multiplier rate. Then when you farm donuts (by buying and selling rat trap trucks), you must have the collider running during the collection to get the most xp. So at first I looked for items with the best ROI, which is about 10 donuts for 1% increase. (I prefer jetbikes and checker-striped bombs) I would run the collider only during the donut farming sessions.

You will want to farm with KeMs first, and you'll want to get your rate above 300% to really see an improvement in donut earning.

Now that my rate is 4000%, I rarely ever farm; but I keep the collider going permanently, because the one time daily rent collection with the Skyfinger monument now generates enough pink sprinklies to get by on.

I didnt know the zirc can be sold and rebought. I will try that.

"4junk3000;c-2208502" wrote:
I didnt know the zirc can be sold and rebought. I will try that.

I was stoked when I found that out. I read it on this forum not long ago and it deserves a credit, but I can't remember who it was exactly.