>They are not for everyone though.
Well, if the dancer/gymnast look isn't your thing, then mebbe
>Pilates (easier) or Yoga (really, really hard)
Hmmmm...not really, whilst there's some commonality you're comparing apples and oranges. (Ok, mebbe quinces...)
Flexibility/balance ? - quite possibly. Strength ? - possibly not. (Probably depends on what variant of yoga as much as anything.) Pilates concentrates on core strength/alignment with stretching to keep muscles elongated and balanced, whereas yoga tends to be a little more about flexibility, with core strength as a result - some/many of the exercises in yoga are intended to massage the internal organs. If you find Pilates easy, then it's probable that you haven't got the appropriate version of the relevant exercise, or you're simply not focussing enough (or else you're a gymnast or dancer). Pilates borrows from yoga/dance/gymnastics and was originally devised as a rehabilitation method (Contrology) - the original 34 'classic' exercises now have many more counterparts to gradually build control/strength (and in some cases avoid injury compared to the originals), and there's several 'schools' (or, to be cynical, franchises). What most people see in gyms is the matwork variant, but there's assorted kit that Joseph Pilates devised, that tends just to be in studios 'cos it's big - but you'll see some versions of it of varying quality on assorted shopping channels.
eg cadillac:
http://pilatescadillac.com/
Cybernight's jobbie is kinda like a reformer:
http://pilatesreformer.com/
Being more familiar with Pilates I can't comment that much on yoga, but way of comparison, have a look at
http://www.easyvigour.net.nz/pilates/h_pilates_classic.htm
and the groups of moves/poses here:
http://www.ashtangayoga.info/practice/asana-vinyasa-series/
..many are pretty similar.
One particularly good thing about Pilates is that, since it was devised for rehabilitation, it accommodates injury fairly well - a lot of people come across it from the result of back pain or injury. If you're having a bad day, pick something easier or with better isolation. I've picked up odd niggly injuries (biceps, tennis elbow) that have kept me from the gym otherwise, but the one thing that's kept me ticking over is an hour or two of Pilates each week (for the last..err 10 years or more).
(As for the swiss ball...the point is to destabilise yourself so you work your core harder to stay balanced. Or to put it another way, it forces you to engage/zip-up your core to maintain stability...nothing wrong with it but it's as much an exercise in balance IMO.)
Hmm... 'manly' and 'larger muscles'.. all very well (in previous years I've spent several hours a week in the gym) but if you can avoid the need to feel macho then, as a cyclist you're better off (IMO) being fit/lean rather than big/bulky - you have to lug the extra weight up hills, after all. Tho' the extra padding's handy if you do much mtbing and crash a lot
You'll get some benefit in your core as a result of lifting weights (you're engaging/zipping up your core at the same time to stabilise yourself) but it's more as a by-product, rather than an aim in itself (tho' I suspect some may disagree). I'd views weights as a complement to Pilates (or yoga), rather than a replacement for. Certainly the breathing/body awareness carries over into doing weights.
Nowadays you'll see a lot people/instructors in the gym doing core stuff that's probably made it's it way over from Pilates (it's been around a long time
http://en.wikipedia.org/wiki/Joseph_Pilates) ..it's not unusual to see otherwise well-built guys doing the relatively easier versions
And *again* - loading muscles isn't the only thing - don't stretch enough and the muscles shorten. Cyclists often have shortened hamstrings as it is, and that in turn can lead to back pain.