We are in Chicago for two nights and have Alinea booked. Planning on trying the Frontera Grill as a walk in on a Friday night for our other option as we left booking too late.
When I asked for the best time I was told 4:30...! Question, is it going to be mad to try at a more civilised time like 8:30pm? If so any other thoughts on options in a similar vein?
Yes, it will be crazy at 8:30, and the wait will likely be well over an hour, possibly 2+, and if a Saturday, you may be told they can not serve you.
Topolobampo is "in a similar vein" in that you enter through the same door, and it is overseen by the same chef/owner. It differs in that it takes reservations, and it is higher priced, and is more refined in that you generally won't fine chile rellenos, tacos, and the like on the menu. When I took my parents from Kentucky there years ago, I described it as "a fine dining restaurant that happens to use Mexican ingredients and flavors, more than a 'Mexican restaurant'".
Other restaurants I would describe as "in a similar vein" based on price point, quality, flavor, and "contemporary-ness", include Mexique, Mixteco, Salpicon, and Mercadito.
Edited to say: I just re-read your post and see you specified a Friday. It will be mad. The after-work crowd will be there in full force, so eating at the bar will be a laughable option, in addition to the usual flocks of foodies, tourists, and other fans.
I agree with all of this excellent advice from GourmetWednesday (and the wait on a Friday at Frontera is indeed likely to be two hours, unless you get there at 4:30, a half hour before they open the doors). Just to add a few comments...
It's difficult to get dinner reservations at Topolobampo on short notice. They start accepting reservations three months out, and they fill up within a few weeks. Lunch reservations are easier, not terribly difficult if it's at least a couple of weeks ahead of time.
Mexique and Salpicon both have the added advantage of being relatively close to the downtown hotel area; Mexique is in West Town, two miles west of the Mag Mile, and Salpicon is in Old Town, walkable from the north end of the Mag Mile. Mixteco Grill is BYO and is about six miles away. Mercadito's menu is less unusual/creative than the others, IMHO.
All of these places (other than Frontera) accept reservations on Opentable.com so you can go to their site and tell at a glance what availability is like for your intended date and time. If this is for tomorrow night, you'll see that availability is wide open at Salpicon and Mixteco Grill, but is spotty at Mexique and Mercadito.
I would not recommend Frontera unless you can arrive extremely early (unless you want a crazy long wait). If you want to experience Chicago's Mexican scene, the best restaurant that likely has availability on short notice is Mexique. While they are a fusion of French and Mexican, it is far more Mexican influenced than French and the food is excellent. They recently were awarded a Michelin star. Los Nopales and Mixteco Grill are also Mexican venues I have enjoyed, but would rank Mexique a step above them.
Tru, Sixteen, L2O and Boka are all outstanding restaurants that often have availability on short notice (they can be booked via Open Table) and would be a nice contrast to Alinea in terms of type of cuisine and overall dining experience. Sixteen has excellent views of downtown Chicago from many of it's tables. Boka is the most casual of the four I listed if you are looking for something more laid back that evening, but still excellent cuisine (and Boka has one of the city's best cocktail programs).
May I suggest you go at 8pm, put your name in and walk to any of the many bars in the two-block radius around Frontera. For dive (and politico hangout) you can go to The Boss Bar, stepping it up is Hubbard Inn, Mercer 113 (a bit 20-something), or Paris Club. Have a drink or two and then come back. Sometimes the host will take your cell number and call you.
Be aware that you won't be able to get a seat at the bar at Frontera Grill; many of the people who are waiting to be seated are happy to sit at the bar (especially since, seated at the bar, you can order from either menu, Frontera's or Topolobampo's).
And your chances of getting seated at Topolobampo without a reservation are pretty darn slim. But if you're standing around the restaurant waiting for two hours anyway, it won't hurt to ask.
If you are in Chicago on Friday morning, you can try the bar at Frontera at lunch. You can usually order from either Frontera or Topolobompo menus (mole negro should be a must if it's available). If you're there by opening (11:30am) you should be able to get a seat at either restaurant, maybe go a bit earlier if you want the bar. The line can start around 11:00am, though it's never been out of control when I've been.
If you still around Sunday morning and are looking for authentic Mexican, give the Maxwell Street Market a shot. Rick Bayless has done a fair bit of shooting for his show at the market so it holds some credibility.
Likewise if you're looking for a lunch, Cemitas Puebla is a good option as well.