Hello (this is in reply to PM, so that others can read it too),
I don't have any experience with these (and would like to buy WRT6
00N for myself, just for the placebo feeling of better range with real antennas and not only some small lines in printed circuits), though I can say some generic things about 802.11n.
802.11n brings the maximum theoretical speed to 150Mbps per one MIMO link (spatial stream). Both of the routers are capable of two spatial streams at once, which means max. 300Mbps. So basically two antennas could be enough for this - however, if the signal reception isn't perfect, the third antenna comes handy allowing for better separation of the individual spatial streams (using reflections etc to its advantage).
Intel 5300 should be able to go up to three spatial streams / 450Mbps, but no Access Point supports it as of now (to my knowledge). For that you'd definitely need third antenna, unless you put the notebook on top of the AP. You should be able to reach 300Mbps (in link speed, not real throughput) with two antennas, but three would give you better range (for 300Mbps and for lower speeds too). If you don't feel like disassembling the LCD, buy some generic 2.4/5GHz antenna and route it under palmrest (as recommended here by bill b. - use the part of palm rest where you don't rest your palm much

)
Theory aside, see benchmarks at
SmallNetBuilder.