i can't speak to ds 6.9 sp6 since we are still on sp5 mr2 but i do know that DS is much easier and more comprehensive than WDS + MDT. in my somewhat biased opinion, the only thing the microsoft solution has over DS is tighter integration with WIM, and the benefits therein (DISM for injecting windows updates, drivers, etc being the biggest advantage of WIMs). sure, DS has image explorer for the rdeploy images, and that's a good thing, but it can't apply windows patches to an existing image the way DISM can, and that right there saves me a ton of time. DS can use imageX to capture and restore WIMs, and therefore you can manually use DISM to keep things updated, but it's a little easier when using a MS product. in SCCM, updating the image is a piece of cake since sccm can also do patching. you can actually set a schedule to have it automatically update itself on a regular basis, so you can set it and forget it and your image will always be fully patched.
that said, i prefer ds 6.9's PXE config much better than WDS. it has a lot more configurability, it can present the user with a pxe boot menu to select which pre-boot environment you want to use (wds/sccm seems to make its own best guess without giving you a choice), and it's more transparent about what's happening. from what little i saw of DS 7.1, its pxe was very immature and completely opaque about what's going on - even worse than WDS. i don't know how pxe looks in ds 7.5 so this comparison may be out of date but this pxe configurability was one of the biggest reasons we kept ds 6.9 instead of ds 7.1.
once you've booted a pc into winpe, the sccm version looks nicer than ds 6.9. if you have ian atkin's excellent ImageInvoker addon for DS, they work very similar, but imageinvoker adds some security and features that sccm lacks - i haven't tried the latest invoker but i understand it now contains AD authentication, so that even if a user boots a machine into winpe, they can't do anything unless their account has permissions to use the DS. not so in sccm - if you can pxe boot a machine, you can image it. well, sccm does have extremely basic security in winpe - you can set a password. whoopty-do! once that password gets out it's all over. granted, invoker only recently added AD authentication, but Ian is just one guy. i'm pretty sure microsoft is not just one guy. invoker also allows you to give the machine a name during winpe, but ns7 and sccm 2012 don't, so you have to handle that on the back end with scripts or something. at my previous job, scripting the computer name was impossible - the naming scheme they used had nothing to do with the computer itself; it was all about its physical location (it was a school district). sccm doesn't even give you the option of doing something like running a vbs or powershell script to prompt the user for a name - it keeps all running processes completely hidden in winpe unless you get really creative with your hacking (and that often breaks the entire process anyway).
i may be a little biased though - i started out doing systems mgmt with symantec livestate (the school district was using livestate well past its EOL), then we transitioned to DS 6.9 sp3 and NS 6.5 (the newest available at the time). i worked heavily in that for a couple years, then changed jobs to another altiris shop running ds 6.8 and ns6.5. i played a big part in the testing, upgrade and decision-making for altiris 7.1. my exposure to microsoft's products is limited to about half a year now of testing and evaluation, then a couple months of production, now that we're completely switched over from ns7 to sccm 2012.
as i said in another thread, we're keeping the DS server up and running because it can do things RIGHT NOW and give you live status of those things as they happen (or don't), whereas NS7 and sccm2012 don't. so even though new machines are getting built with sccm 2012, we're still installing dagent on them too.
DS 6.9 has spoiled us and i will truly miss it when they finally make us decomission that server.