My field has about 20-30 people on a Saturday, 40 on a good day. Kinda hard to find 2 clear channels with 6 people up at a time, all using 2.4.
How do you figure? There are 80 channels in the band. 6 people flying, probably another 4-5 in the pits working on their planes, your radio will be fine. In the testing that was done when dsm2 first came out they had over 30 transmitters on simultaneously and had zero problems. The only times I've heard of problems due to not having channel availability and thus a lockout was at really large events like SEFF and IRCHA that have over 200 registered pilots with tons of them in the air at the same time. Both of those events have multiple flight lines and likely 30+ people in the air at the same time.
I've used dsm2 for about 4 years now, and have been to numerous events in our area. I frequently fly dsm2 with 5-10 other pilots in the air on 2.4ghz and have no issues at all. I've been to a few events with 25 planes flying at once, a mix of 2.4 and 72mhz, but at least 15 of them were 2.4ghz. No problems. I wouldn't worry a bit about having 6 people on your field at once. DSM2 or DSMX will both work fine.
Now as for the dx6i... another not mentioned disadvantage I've experienced with it is latency, it's just plain slower than other radios. I've flown tons of dx7's, I used to have a 9303 and currently have a 9503. Occasionally I buddy box a new pilot at our field on dx6i's and they feel slow.
It will definitely work, and is a lot better than your current dx5, but eventually you will be upgrading it. If it's all you can get now the upgrade is worth it, especially at the price you mentioned. Just realize that it has limitations and you will eventually want more...