i am working with both for some years now, and i am certified as "high availability designer for unix"... on my own opinion, i like the integration of the SunCluster more than VCS. like blowtorch said, the SunCluster is a kernel cluster which gives you very fast error detection, VCS just monitors the environment with "scripts" and daemons. to say that VCS uses "no kernel-level integration" is not true at all, because the cluster communication uses two kernel modules called "gab" and "llt"; anyhow, these two modules comes with the VCS but can be also found at some other products..
VCS has a nice user interface (GUI) and is "easy" to manage, where the SunCluster is a bit more "unix like". it's true to say that VCS was one of the best cluster solution in the past, comparing it to SunCluster 2.2. but since SC 3.0 the SunCluster became better and better. Looking at SC_3.1-U4 vs. VCS 4.1 i would miss nothing in SC what i need in a cluster environment. The latest version of SunCluster is 3.2 (released in 01/07), which brings a lot of new features. no question, VCS 5.0 is a massive product, but i when i am reading the release notes of VCS, i always think about a big table where 100 product managers think about new features no one really needs; reading the SunCluster release notes reminds me on 100 engineers planning how they could increase the availability of a cluster environment.
another point to think about are the costs, with VCS you must buy VxVM aswell, and every nice feature is an extra license (best example io-fencing) licenses based on tier levels... SunCluster could be used for free inside the Java Enterprise System without support... if you buy SunCluster, you get all features it would provide for one price... the SunCluster uses standard features from solaris like IPMP, MPxIO, UFS or SVM/LVM, with VCS you get thinks like VxVM, VxDMP, GAB/LLT or even VxFS
at the end of the day, i like to get support for all products from one hand, there can be no finger pointing in extreme emergency....

but once again, many of my costumers are very happy with their VCS and many are very happy with SC... It's purely a matter of taste....