This is a subjective and argumentative question and thus should be closed - you don't provide any info about your startup so it's hard to provide a concrete answer, so you'll only hear subjective opinions from either sides of the camp :)
That being said, let's get some things straightened out :) GWT and PHP can work together (and do nicely too - I've used PHP in my previous GWT project). You use GWT for your frontend, while PHP for the (server) backend. Communication is usually done via JSON. GWT provides a very nifty thing called JavaScript Overlay Types that allow for a very smooth (and fast, since it uses browser for the parsing) integration in your GWT app of the JSONs you get from the server. Also, if you're an (experienced or not :)) PHP programmer, UiBinder should be a breeze to learn and it offers a familiar environment to create templates for your widgets.
With the advent of GWT 2.1 however, it might more advantageous to stick with "pure" GWT, for client and server. This is mostly because GWT 2.1 does a lot of work for the programmer when it comes to communication between server-client (caching, auto-updates Comet style, etc). See this presentation from Google IO 2010 for more info.
Oh, and PHP and GWT as a whole aim for different things: PHP is for sites with dynamic content (so most "normal" sites nowadays), while GWT is best for creating RIA (Rich Internet Applications - think GMail and the like). While you can create RIA with PHP, it's cumbersome - the same goes for GWT and "normal" sites (you'd rather just create small widgets in GWT to embed into a normal, maybe PHP generated, site).
PS: One think that is definitely (IMHO, of course) that GWT has advantage over PHP is language. It's a matter of preference, but I prefer strongly typed, object-oriented language like Java, over PHP, that's trying to be many things at once (adding object-oriented functionalities, some info about types, etc.) - the one thing I didn't like about my GWT+PHP experience was the PHP language - maybe it was just my lack of experience in PHP, but I just didn't like the type of code PHP made me write.