A couple of points to think about. One, you may want to check out the website astromart.com (a $15 fee per year) or the Cloudy Nights Classifieds (free). 8 and 6 SE scopes come up for sale quite a bit (not because they are not good, but because a lot of people buy them). If you are looking to save money that's a possible way. I've seen 6SEs go for $500ish on the used market (just be careful for shipping/paypal fees).
Between 6 and 8 inches for an SCT, there is a bit of a difference. But it's not too significant. At a $400 difference, I may be inclined to get the 6SE. You could quite possibly be able to get an 8 inch OTA later if you felt the need and sell your 6 inch OTA. I've seen 8 inch SE OTAs go around $400-500 used. Then your 6 inch OTA would fetch $300-400.
I've looked though both and both were pretty good. The 8 inch would be a bit brighter on deep space objects. The 6 SE would probably cool down faster (important for these scopes to be cooled down for best images). Cool down can take up to an hour or so depending on the difference from inside to outside on the 6 SE.
The biggest potential difference is 6 inch SCTs are basically limited to 1 and 1/4 inch eyepieces (not necessarily a bad thing). Whereas you could pretty much utilize a 2 inch setup with an 8 inch (extra money to get that all setup though).
If it were me I would get the 6SE. Spend the extra money on a little nicer Diagonal and eyepieces. If the cost used was $500 shipped, that's not bad. If it was plus shipping, it'd be a bit harder to buy used (unless it had some bonus stuff with it).
An alternative to the SE line, and a bit lower cost would be an 8 inch Dobsonian telescope. For under $400 new you can have a very capable scope. But it would not have "go to" (to help you find an object). You'd still want to add a few things to that like a tool for lining up the mirrors (around $40). The 6 inch is OK also at around $300, but it (like the 6SE) doesn't have a 2 inch focuser. Orion Telescope telescope.com sells these. There are other companies that have dobsonians as well.
That being said, I think the 6SE may be better given the electronics and tracking (following the object as the earth turns). The 6SE would be lighter weight (not by too much). The 6SE looks a little better to me, but that's subjective and not really a big deal probably.
On the used market it's tough because of shipping. But if you can find one locally (I hate to suggest Craig's list because of security, but it's a possibility) you can find good deals. Most beginners start with either the SE line or the 8 inch dob. Both are good scopes...so it just kind of depends on if you want electronics or not. I personally like electronics in some ways, but I can also see the benefit of larger aperture for less money.
Just some things to consider.