This is kind of tough cause you're comparing apples and oranges. WinStep is very specific. Its aim is to be extremely productive via a popup menu system and tabbed program launcher. Object Desktop is a collection of apps, the most popular of which are free form desktop enhancements. It also depends on how you use your desktop.
WinStep Pros
NextStart is the most useful popup menu system I've seen. Hotspots make it extremely flexible. NextStart really comes into its own when you use multiple menus.
Extremely easy to create menus (an internet menu, an apps menu, development menu...)
WorkShelf is a really nice tabbed program/shortcut launcher. It can be docked or free floating and made to appear on screen bump.
Themes are immediately applicable. No tweaking involved. If you want to change themes alot then WinStep is the way to go. Its all fire and forget.
WinStep Cons
NextStart's menus don't have a MFU (Most Frequently Used) feature or the expanding/collapsing recently accessed menu items that ObjectBar and does.
Task management and sys tray management not as full featured as ObjectBar. Tasks are only shown as tiles with no text (but hey, you don't have to use NextStart's task management if you don't want to).
Object Desktop Pros
Has a tabbed program launcher but I've never used it so I really can't comment on it.
ObjectBar real nice task/dock/wharf program. Graphically more flexible than NextStart or WorkShelf. Its more akin to HoverDesk.
ObjectBar's Task management and systray management are nicer. Can filter your tasks and items in the sys tray. VERY useful.
DesktopX allows you to have interesting and ever more useful objects on your desktop, especially now with the advent of DX scripting, say a news ticker or a web search object
Object Desktop Cons
Since ObjectBar and DesktopX are more free-form than WinStep, themes have to be tweaked/modified to reflect your use.
Most ObjectBar skins are currently just stylistic variations of the Windows XP StartMenu and taskBar. So if you use these rather than design your own theme, you're just using the
same Windows taskbar paradigm. If you like that great. If not, oh well...
As menuing system, ObjectBar is a little harder to configure than NextStart. Remember, its more of a dock than a menu system.
Unlike NextStart, ObjectBar's auto-hide bar feature immediately displays bars immediately rather than having a configurable delay. EXTREMELY annoying. If they ever fix this and add configurable edge detection delays I'd go wild with making new bars and use ObjectBar a heck of a lot more than I do now.
With all that said, in the past I've usually used WinStep because I don't like having to modify each theme I install. Additionally, I usually run my apps in maximized window mode. NextStart has a nice bump-screenedge/screencorner hotspot functionality that allows me access to all my menus.
DesktopX is nice but the problem here is that since I've got windows covering the desktop, I have to minimize windows just to get at DesktopX.
Being the inconsistent person that I am, lately I've been running ObjectBar and DesktopX for variety. I really like ObjectBar's task/systray managment. But more often than not, I go running back to WinStep for its menuing abilities.
It would be nice if WinStep had a better task management/wharf system and I believe once is in the works.
If you forced to choose one or the other based on productivity, it'd be difficult to choose one, but I'd lean towards WinStep.
But luckily I don't have to choose. There's nothing preventing you from running some combination of WinStep apps and Object Desktop apps. I do that all the time.