I think you'll have a quality race in either so I guess if would depend on whether or not you want 2 more weeks of training or not. Stanford is definitely deeper though. They had 3 sections, the 3rd section was one of the first events on friday, the 2nd was one of the last on friday, and the 1st section was the last race on saturday. I don't know what the time standards are for each heat but here are results of people close to/near sub 29 from last year, found on trackshark:
Mt Sac (1 section, 49 people total)
1 Songok, Shadrack Tex. A&M-Corpus 28:16.47
2 Coolsaet, Reid Reebok 28:21.86
3 Olmedo, Pablo Unattached 28:22.37
4 Bizuneh, Fasil New Balance 28:25.69
5 Sundell, Steve Unattached 28:28.01
6 Romero, Juan Mexico 28:38.41
7 Bodor, Oliver Western State 28:44.71
8 Gildea, Patrick Nyac 28:45.68
9 Johnson, Jeremy New Mexico 28:47.76
10 Solis, Jesus Adams State 29:06.98
Stanford (1st of 3 secions, 26 people total)
1 Nicodemus Naimadu SR Abilene Christian 28:06.26
2 Stephen Samoei SR Utep 28:07.65
3 Ryan Hall NA Asics. 28:07.93
4 Jason Hartmann Nike- 28:15.22
5 Kevin Chelimo SR Texas Tech 28:21.29
6 Japheth Ng'ojoy JR Utep 28:24.48
7 James Carney NA New Balance. 28:25.24
8 Andrew Lemoncello SR Florida State 28:28.04
9 Ed Torres Reebok" 28:28.30
10 Sean Quigley La Salle 28:29.16
11 Matt Downin NA New Balance- 28:30.53
12 Galen Rupp JR Oregon 28:35.04
13 Adam Goucher NA Nike- 28:43.80
14 Ian Burrell JR Georgia 28:48.78
15 Eric Gillis NA Speed River Tfc 28:52.57
16 Kevin Castille NA Team Eugene 28:53.78
17 Andrew Middleton SR Texas 28:54.87
18 Keith Bechtol SR William & Mary 28:55.57
19 Jeremy Mineau SO Washington 28:57.02
20 Chris Seaton NA Reebok; 28:58.19
21 Trever Ball NA New Balance 29:00.44
22 Shadrack Kiptoo-Biwott SO Oregon 29:00.52
23 Bret Schoolmeester NA Nike- 29:03.89
Stanford (2nd of 3, 29 people total):
1 Powers, Jeff SR Central Michigan 28:43.83
2 Khadraoui, Mohamed SO Iona 28:49.08
3 Rono, Aron SO Azusa Pacific 28:50.81
4 Bauhs, Scotty SO Chico State 28:54.55
5 Hoerr, Trent JR Illinois 28:55.22
6 Kinsella, David SO Portland 28:57.97
7 Dumm, Andrew U of Virginia 28:59.25
8 Batliner, Payton SR Colorado 29:00.33
9 Kirui, Barnabas SO Ole Miss 29:16.39
Winner of 3rd section (18 people total):
1 Ogwaro, Oscar SO East LA College 29:41.30