Personally I prefer the Spillers. I used the Bailey's for the first 2 winters I had T and have switched to the Spillers because it's cheaper and, for T anyway, seems to work better. I checked the ingredients and they're very similar to most conditioning mixes/cubes. He's very shiny and doesn't need any other supplements, I feed them with cherry chaff, sugar beet and carrots.
Personally, I've always had very good results from any Baileys product I've used and can't say the same for Spillers!
Have you thought about their stud cubes as they're very good for condition?
Out of the two I'd go for Baileys every time. Last winter one of our wussy warmbloods dropped a lot of weight - we used Baileys No 4, Alfa Oil and Sugar Beet to get it back on. I think sometimes the difference is also how often you feed - I feed all mine 3 times a day (which I know isn't possible for everyone) but I feel that 3 smaller meals rather than two bigger ones must be easier on their system and they can digest it more easily.
I'll be honest I have him on Baileys with very good results. I only bought one bag so I could make sure he didnt blow up on them. Phoned up to get a couple more and merchant wont have anymore in until Tuesday so I was going to use Spillers as he has some left. He's had a sudden run on feed apparently! And yes, if I knew I was going to stay on Baileys I would always have at least a bag in hand but I didnt and I havent.
The Spillers contains 12 MJDE/kg whereas the Baileys is more calorific at 13.5 MJDE/kg, so if you have to switch to Spillers temporarily you would need to feed a bit more by weight if you want to ensure the same calorie intake you were getting with the Baileys.
I prefer Spillers, I did try the No 4 but they sent my CB bonkers (and nothing ever heats him up including the cheapest and most sugariest mixes). He was completely normal on the spillers cubes and he really filled out (he was a baby at the time going through a growth spurt).
I used to use no 4 and they were very good, then our local feed merchant stopped stocking them as they are v expensive. They now have their own brand of conditioning cube which is much cheaper and in my opinion does the same job - CWG equiclub conditioning cubes.
I personally prefer Spillars - do the job and never had the horse getting fizzy on them - did with the No 4 - nothing major, but Spillars for mine - from TBs to cobs has been the safest bet.
I've never used spillers... but they contain less MJ per kg than baileys... and I'm having fantastic results with baileys at the moment for keeping weight on, so daren't change.