Many people, myself included, are of the opinion that a secondary isn't really necessary. In fact, leaving the beer in the primary longer has more chances of the yeast cleaning up any off flavors that beer may have. The only time I used a secondary is when I did a fruit addition. Otherwise, you increase your risk of oxidizing your beer, contaminating your beer, but more importantly, you're making more work for yourself. I'd say if you really, really wanted a super clear beer with no sediment, then you can still go for it, but you're just going to be priming it and bottling it anyways and will always end up with yeast in the bottom of the bottle.
If you still MUST get a secondary and you want to use a carboy, get the 5 gallon version to reduce headspace thus decreasing the surface area. If you're like me, I would just get a second 6/6.5 gallon vessel that you can secondary in if you need to but can use as a primary any other day of the week. Just my $0.02