I actually own both of these printers, the i9900 was my primary printer until I found a smoking deal on the 3800, with the couple of test prints Ive done there's no denying that on the same 'gloss' paper Ive always used with the i9900, the i9900 blows away the 3800. The i9900 prints look wet compared to the 3800, now with that said, these same test prints have a B&W section that look terrible, on the 3800 much better. I'm not an expert by any stretch of the imagination but it would seem as if the dye inks were made for high gloss type papers and they soak right into them where the pigment inks don't, I dunno.
What I need to do is find various types of papers and see what works best with the 3800, I'm really curious to see if there is a better suited high gloss paper that works much better with the 3800 so I could really compare it to the i9900 prints.
Regarding B&W prints, imho, the 3800 will smoke the i9900 on just about every type of paper, mostly due to the color casts the i9900 is known for.
I'm definitely going to try more tests over time, I especially want to compare prints on some pearl and/or matte paper, from what Ive been reading there's no way the i9900 will compare. Ive printed on some a b&w and color print on some PhotoRag 308 on the 3800, the results were amazing, I'm too new to these kind of papers to know if I can do a sample print on the i9900 but if they accept dye inks I'm going to have to give it a try just to see how they rate to one another.
So, in my opinion, don't rush the purchase of a 3800/3880 unless you find a great deal for a used one.