Thermo Fisher Scientific (NYSE:TMO) and Illumina (NASDAQ:ILMN) are two companies investors interested in genetics will want to keep a close eye on. Both stocks have done very well over the years, currently trading near their all-time highs. Although they’ve done well in the past, determining which one is the better buy here on out is a more difficult question to answer — one that requires assessing where the companies are today, their relative valuations, and overall risk level.Good diversification can make Thermo Fisher Scientific an attractive investment, but the company’s level of growth may not be that high for growth investors. Over the past nine months, the company’s consolidated revenue has increased by just 4.8%. The life sciences segment led the way with sales growth of 9.8%, but laboratory products and services were up 4.5% and the other segments showed less revenue growth. That level of growth makes it hard to justify Thermo Fisher Scientific’s 36 price-to-earnings (P/E) multiple. Even its price-to-sales (P/S) ratio of five is a bit high. For dividend investors, the company’s dividend yield of 0.23% is far below the 1.85% investors can find with the S&P 500 index.